[Author’s note: This article assumes that there are two thousand pounds to the ton, and a standard construction brick with pointing is 9″ x 4.5″ x 3″ (121.5 cubic inches) and weighs eight pounds.]
Making bricks is easy you say. Mankind has been making them for millennia. You dig up some clay, mold it to the desired shape, and then fire it until it is hard. Easy, straightforward, anybody could do it. Right?
Wrong. Problems can occur in the preparation of the clay, the molding of the bricks, the drying of the bricks and the firing of the bricks. If any step is not performed correctly, then the finished product will be unsatisfactory.
Take the early European colonies in American as an example. In 1633 Wouter Van Twiller, the governor appointed by the Dutch West Indian Company, started construction of his private residence on Manhattan Island. This fact is interesting not just because it was the first brick building in America, but that the bricks were all imported from Amsterdam. This suggests that the colony was unable to produce good quality bricks. We find that fired bricks probably weren’t produced in America until 1650 when the New Haven colony fired their first bricks. It’s not that the colonists didn’t try to make bricks earlier. Rather, the problem is that until New Haven in 1650, they didn’t have any brickmakers, and the earlier attempts resulted in inferior bricks that were mostly unsuitable for construction. Having people who know what they are doing is important for the production of useful construction bricks. This is a problem facing the people of Grantville. Brickmaking in the Grantville area of West Virginia died out in the late nineteenth century, so there is little chance any current resident has sufficient experience of brickmaking to be useful. This means that Grantville will be depending on the skills of down-timer brickmakers.
This presents a new problem. Brick is not a popular construction material in Thuringia, the area in Germany where Grantville has been deposited by the Assiti shard. This means that there will be few brickmakers near Grantville, but more importantly for the short term, there will be no infrastructure in place for the production of bricks in volume. There will be no large stockpiles of clay dug up last autumn and left to weather over the winter. There will be no drying sheds, nor will there be permanent kilns. Worse still, there will be insufficient dry wood, the fuel of choice until the nineteenth century, available for firing bricks. Any brickmakers in the area will be refugees or itinerant brickmakers moving from job to job. Either way, without up-time assistance, they will be unable to produce bricks in any volume until the brickmaking season of 1632.
The brickmaker is responsible for the following tasks:
Preparation of brick earth.
Molding of the bricks.
Drying of the freshly molded bricks.
Firing of the dry bricks.
1) Preparation of brick earth
Earths (the technical term used to refer to soils as opposed to rocks) suitable for brickmaking fall into three principal classes:
1) Plastic or strong clays, which are chiefly a silicate of alumina. Often called Foul clays by the workmen because of the odor they give off, they are also known as Pure clays
2) Loams and mild clays are those with a considerable proportion of sand intermixed.
3) Marls or calcareous clays are, as the name suggests, clays containing a notable proportion of carbonate of lime.
As both the Grantville area of West Virginia and Thuringia abound in suitable earths a down-time brickmaker will easily locate clay suitable for brickmaking within or close to the Ring of Fire, He will then have to prepare it for molding. It is rare to find naturally occurring clay that is suitable for brickmaking. Thus the preparation of clay becomes an important step. Our skilled brickmaker will examine the available earths and decide what has to be added to it for successful brickmaking. The pure clays require the addition of sand or loam, while the loams often need the addition of lime to flux and bind the earth. Pure clay or clays with little sand content will shrink and crack while drying no matter how carefully and slowly the bricks are dried. They will not stand firing, as a red heat will cause the mass to rend and warp. To overcome this problem substances that do not combine with water and do not contract when heated are mixed with the clays. It is common practice to mix in ground-up burnt clay (grog) from failed bricks. This is especially true when making fire bricks, as the fire clay tends to be expensive. For example, considerable savings can be made by mixing two parts by weight of burnt clay to one part Stourbridge clay to produce a good firebrick.
Once the brickmaker has located a suitable supply of clay and decided what to add to make it suitable for brickmaking, he has to extract the clay and then prepare it. Digging the clay and transporting it to where the bricks are to be made and fired is going to be labor intensive. It takes about two cubic yards of clay to make one thousand bricks. A single palletmolding team consisting of a molder and three or more assistants can mold and lay out to dry something like three to five thousand bricks per fourteen hour day. Just to keep one team working, the clay diggers are going to have to dig six to ten cubic yards per day of brick making. A single worker can dig wet clay (as opposed to mining dry clay) at about fifteen cubic feet per hour. At this rate, ten cubic yards requires eighteen man-hours of digging.
Before the introduction of large-scale mechanization, clay was usually dug in the autumn. It was then transported to a level place prepared to receive it and left in heaps several feet high to weather over the winter. The winter frosts would help break up and crumble the lumps. Clay that has been dug in the spring will not have benefited from the frost action and, apparently, will produce inferior bricks. The object of the weathering process is to open the pores of the clay and to separate the particles so that the clay can absorb water more readily when it is mellowed (made pliable and plastic).
At the start of the next brickmaking season, which normally starts in April, the clay is removed from the heaps and thrown into treading pits where water is added, and with a combination of spade labor and treading by barefooted humans and animals, the clay is tempered to the desired plasticity. Throughout the tempering process any stones that may be found in the clay must be carefully picked out by hand. This is a tedious and time consuming operation, but one which cannot be neglected, as the presence of a pebble in a brick will result in an unsatisfactory brick. Usually the different density of the pebble compared to the clay will result in cracking of the brick.
For earths that contain a lot of gravel the only option is to wash them and run them over a grating so sufficiently fine that not even small stones can pass through. The liquid pulp has to then be run into a pit that is prepared for it and left until sufficient water has evaporated. This process produces clay that is perfectly uniform in texture throughout the mix. However, it is expensive to prepare and therefore to be avoided unless the bricks you are making require the perfectly uniform texture, such as in cutting, or rubbing bricks. Rubbing bricks are laid with almost no mortar between them, and are “rubbed” or cut to give a very close fit, such as in gauged arches.
When working with marls care must be taken that no lumps of limestone survive the tempering process. Even a piece of limestone no bigger than a pea in a molded brick is sufficient to destroy that brick. The carbolic acid is driven off by the heat of the kiln and forces a vent through the side of the brick. This creates a cavity that water can enter. The first sharp frost this brick is exposed to will freeze water in the cavity and will generally be sufficient to destroy the face. Over time the entire brick will disintegrate as the exposed area takes in water that will freeze and thaw.
The people from Grantville can contribute to the preparation of the earths in a number of ways. Principally, their contribution will be finding ways to mechanize the processes. When it comes to digging the clay, the introduction of mechanical diggers will not only increase the rate at which clay can be dug and loaded onto wagons and carts, but it will also extend the digging season. In seventeenth-century Europe the earths are dug in the autumn for a number of reasons. Probably the most important reason is that autumn digging allows weathering over the winter. During the summer the laborers who could be digging clay will be busy tempering the previous year’s clay. This leaves only autumn and winter for digging. Winter digging is not desirable as not only does it reduce the amount of weathering the clay can undergo, but also there are the problems associated with trying to dig in wet and maybe frozen earth using wooden or poor iron shovels. Not only will the laborers have difficulty digging, but also standing in wet earth under winter conditions for hours on end will damage their health.
Mechanization of the digging process means that we now have earths being delivered to the brickworks right through the year. This means that not all the earth will benefit from weathering. We know that this will result in inferior bricks. Therefore something has to be done to replicate the weathering process. The easiest methods of breaking down the earth, and for that matter, any small stones and clumps of limestone, is to use the grinding mill.
Figure 2. A single pair of rollers for crushing clay. g and g’ are the two counter rotating rollers. h is the feed hopper, i is the separation distance between the rollers. (Dobson)
The simplest grinding-mill consists of a ring-pit around which a draft animal drags a heavy cast-iron roller (figure 1). Clay is spread around in the pit and the roller is repeatedly rolled over the clay. Water and additional earths such as sand are added until the clay is the desired plasticity. Men then empty the pit with shovels. Because the pit is out of service while being emptied it is useful to have more than one ring-pit, allowing grinding to continue. An alternative to the ring pit is to use pairs of counter rotating cast-iron rollers arranged similar to an old-time wringer or mangle to crush clay, stones, and lumps of limestone (figure 2). It has several advantages over the ring-pit: first, it uses much less land, and second, it is an all-in-one process. Instead of loading the pit, grinding the clay, then unloading the pit, the pairs of counter-rotating rollers allow raw clay to fall through the machine, compressing the clay and crushing anything larger than the separation gap between the rollers. With multiple pairs of rollers, the separation gap can be reduced pair by pair until the final pair is almost touching. The resulting product is not as good as that from a ring pit, but it is usually good enough for most purposes, and more importantly, it is cheaper than using the ring-pit. Clay going through the counter-rotating rollers will still have to go through a pug mill, but by placing the pug mill below the roller arrangement the clay can fall directly into the pug mill, greatly reducing the amount of handling, and thus the cost of brickmaking.
The pug mill can either be used on its own, when earths are known to be free of stones and limestone lumps, or used in conjunction with the grinding mill. A pug mill is usually a cylindrical vessel with a central shaft that holds a number of knives, which by their motion, cut and knead the clay. The knives are arranged so that the clay is gradually forced through the mill so that the finished product is thoroughly tempered ready for molding, and is similar in principal to a kitchen mincer. It is possible to have twin-shaft pug mills. The counter-rotating shafts temper the clay more efficiently than a single shaft, but the arrangement of the shafts, and the casing surrounding them are more complex. Energy to rotate the rollers of the grinding mill, or the shaft or shafts of a pug mill can be provided by draft animals pulling a sweep, water wheels, windmills, or some type of engine, either an internal combustion engine or a steam engine.
2) Molding of the bricks
There are two methods of hand molding that may be current in seventeenth-century Europe. They are slop molding and pallet molding. The difference between the two is principally in the release agent used to prevent the clay adhering to the mold. In slop molding the mold is dipped in water from time to time, while in pallet molding the mold is sanded, rather like flour is used as a release agent in baking. Generally the slop molder needs at least two molds, while the pallet molder makes each brick with the same mold. It appears that there is little difference between the two methods in terms of product quality, and except for having a wetter surface on the brick, which means it can’t be immediately tipped out of the mold and then hacked, there is little difference in how the bricks are handled. The following is a description of pallet molding.
The pallet molder makes his new brick by throwing a clot of clay forcefully into the mold using a two-handed throw. Using the hands to force the clay into the corners of the mold is undesirable, as it changes the density of the clay in that area and any uneven pressure exerted in forming the brick will show up as distortions in its shape as it dries or is fired. The molder then uses a strike to level off any surplus clay before turning out the molded brick onto a pallet. The pallet, a small board slightly larger than the brick, is then put to one side for an assistant to put onto a hack-barrow. When the barrow is full it is wheeled off to the drying ground where the bricks are stacked for drying (except for the case of bricks to be fired in a clamp or scove, where the bricks are taken to the hacking-ground and stacked into hacks). Our single pallet molder can keep two people wheeling barrows (wheelers) constantly employed, with two barrows being always in work while a third is being loaded at the molding stool. The drying floor for a brickworks producing over thirty thousand bricks a week can cover a large area, so it is not uncommon for bricks to be wheeled over fifty yards from the molding stool to where they will be stacked for drying.
In wet climates such as can be expected in Germany, the drying area should be under cover to protect the green bricks from rain or sun. The direct impact of rain can break up bricks, or at least cause uneven drying, while direct exposure to the sun can result in uneven drying, which can cause the brick to distort as it dries. Walls on a drying shed help to prevent unequal drying caused by uncontrolled air currents, and when a heated drying shed is used, the walls help retain the heated air.
A slop molder follows almost the same procedure, except that the molder doesn’t tip the brick out of the mold onto a pallet. Instead, an assistant takes the mold to the drying floor and tips it out there before returning with the mold. The effect of this is that the distance between the molder and the drying floor can’t be too far, otherwise the molder is left waiting for a mold, or too many people are employed to carry away molds for it to be economical.
The first improvement Grantville can bring to brickmakers is the introduction of a kick to the bottom of the mold (figure 3). Also known as a frog, the kick is a rectangular block of wood or metal, smaller than the mold dimensions, that is screwed onto the bottom of the mold. Sometimes letters were carved in the kick to identify the brickyard owner. It is a raised area on the bottom of the mold that creates a hollow in the newly made brick. When the clot is thrown into a flat-bottom mold it sometimes fails to fill the edges where the sides meet the bottom. The molder then has to take remedial action when the brick is turned out onto the pallet, taking up valuable time and reducing production. How does the kick force the clay from the thrown clot into the corners? A clot of clay hitting the flat bottom of a mold with force can go no lower, so the clay starts to spread out towards the sides. There may not be sufficient force applied to the clay to get the clay into the corners of the mold, especially those at the bottom of the mold. The kick encourages the clay to flow into these bottom corners as it can still move down even after hitting the kick. The presence of the kick reduces the number of occasions when the clay fails to reach the edges, increasing the number of good bricks made in a given period. I cannot confirm that the kick wasn’t in use in the seventeenth century, however, there is evidence that even as late as the late eighteenth century American brickmakers were still hand molding bricks without a kick.
Figure 3. A Brick mold with bottom plate. Note the raised kick. The pins ‘a‘ are to secure the bottom plate to the molding table. (Dobson)
Mechanization of brickmaking can come from two directions. Either the throwing of the clots can be mechanized, or bricks can be extruded. Mechanical clot throwing is a relatively modern technology and is probably beyond the capabilities of Grantville for many years. In the meantime, there is nothing mechanized clot throwing can do that can’t be replicated by employing more hand molders. Extruded brick though, that offers new opportunities. A brick extruder nozzle can be attached to the end of our existing mechanical pug mill. Clay will then be extruded in the shape of the nozzle as a continuous block of tempered clay. In our case, a block nine inches wide and four and a half inches high. This continuous block of clay can then be cut into three-inch slices using a thin wire. The bricks are then placed on a pallet and transported to the drying shed. An extruder made out of cast-iron and weighing about four tons powered by a ten horsepower steam engine is capable of taking raw clay and pumping out up to ninety thousand bricks a week. Which looks good, until you realize that is about the same as three molding teams. There is a saving in labor, but in seventeenth-century Germany labor isn’t exactly rare or expensive. So why would you want to make a considerable capital investment in a combined grinding mill, pug mill, brick extruder?
There are two reasons. First, the mechanical brickmaker tempers its own clay. The second reason, and one that makes developing extruder nozzles worth while, is the manufacture of “hollow bricks.” Hand molders can’t easily make bricks with holes, while an extruder just needs a slightly modified nozzle to go from solid brick to hollow brick. Hollow brick will be in demand for several reasons. The hollow bricks can be for ventilation, or insulation, or steel reinforcing can be threaded through the holes. Whatever the reason, for the same volume of wall, hollow bricks will be considerably lighter than solid bricks. The lighter brick is less fatiguing to handle, and mortar adheres to the textured surface better than to a smooth surface. These two characteristics make the bricklayer’s job easier. There are savings to the brickmaker as well. First, less clay is used to produce the same volume of brick (the space occupied by the brick). Second, the hollow nature of the brick means that no particle of clay is now much more than half an inch from the heat. The heat now only has to penetrate about half an inch, as opposed to an inch and a half for a solid brick. This means bricks naturally dry and fire faster. This leads to a saving in fuel to fire the bricks. The savings in clay, and the reduction in firing time and the fuel required to fire the hollow bricks produces considerable savings and can significantly reduce the cost of making the bricks. Also, the same extruder technology can be utilized to form concrete blocks once we start using concrete for construction.
Of course things aren’t going to be easy. As the clay is extruded through the nozzle the edges of the clay tend to catch on the edges, and especially the corners, of the nozzle. This results in bricks that distort as they dry. The answer is, of course, to lubricate the clay with a little water as it passes through the nozzle. For our heroes in Grantville, learning how to do this will be an exercise in trial and error. Assuming they even know what the problem is.
An additional benefit of the extruder nozzle is that, once we have it working properly, we aren’t restricted as to its shape. This means we can easily convert our brick extruder to extrude other products. Realistically, we can use a single machine type to make different sized bricks, roofing tiles, and sewer pipe, just by supplying a range of extruder nozzles.
3) Drying of the freshly molded bricks
Freshly made bricks are referred to as “green’ bricks.” Usually they will contain too much water for immediate burning in a kiln. This water has to either dry off naturally before they are put into the kiln, or valuable fuel is consumed drying the bricks in the kiln. For this reason, green bricks are usually set out on a drying floor or in a drying shed where they are allowed to dry uniformly to the point where they can be safely handled without damaging them. They are then hacked (more on kilns and the different types in the next section). Hacking involves taking the bricks from the drying surface, where they are only one deep, and stacking them edge on edge in a hacking ground for further drying. How bricks are dried before being placed in a kiln depends on how they are to be fired. Bricks that are to be fired in clamps or scoves are usually pallet molded and hacked straight off the hack-barrow rather than being set out on the drying floor. These bricks must be drier than those to be fired in a kiln. This is because clamps and scoves attain their maximum heat almost immediately and cannot be regulated. So anybody intending to use clamps or scoves to fire their bricks must ensure the bricks are properly dry. This can mean a stay on the hacking floor ground of several weeks.
In Germany it will probably be impossible to air dry bricks throughout the year. For this reason heated drying sheds should be used. A heated drying shed has walls and a roof to keep the heat in. It also has a heated floor. Hot air is sent along the floor, and sometimes up the walls, through vents. Ideally we want sheets of iron over a floor of channels, much like Roman central heating. The hot air heats the iron, which heats the air in the drying shed, which in turn warms the bricks, drying them. A drying shed will require a fire to heat it, although waste heat from kilns might be used.
4) Firing of the dry bricks
Why do we have to fire bricks? What is wrong with sun dried bricks? Water is the problem. Water can either wash away the clay, or crack open the brick when it freezes. The objective in firing bricks is to create a hard brick with a weather resistant finish. Terra-cotta bricks can be made, but they will lack the glaze of silicate of alumina based bricks, and will need a second glazing firing to make them weather resistant.
The burning or firing of bricks is the most important factor in brickmaking. Their strength and durability depend on the style of firing and the degree of firing to which they have been subjected. Firing is supposed to bring about certain chemical decompositions and recombinations that entirely change the physical character of the dry clay. The finishing temperatures (the temperature that the bricks must be exposed to for them to fire properly) range between 900° C to 1250° C (1652-2192° F), with a usual temperature of about 1050° C (1922° F) for ordinary construction bricks. Fire bricks need something like 1250-1500° C (2192-2732° F).
Table 1. A list of different methods of firing bricks, giving their fuel consumption to fire 1000 bricks. Methods are ranked in decreasing proportions of over or under cooked and broken bricks.
The brickmaker brings knowledge based on experience to the firing of bricks. The first task is to correctly arrange the bricks within the kiln (known as setting the bricks). Bricks have to be carefully stacked in the kiln to ensure an even distribution of heat. This means bricks have a finger-width space between them. Bricks are stacked in pairs with faces in contact. This is done to produce clean-surfaced faces for cosmetic reasons. If bricks were arranged in rows with each layer lying perpendicular to the other, then the hot gases roaring through the gaps between the bricks would leave patterns on both faces of the brick. By placing a second layer of bricks exactly on the top of the previous row, every brick will have at least one face that wasn’t exposed directly to the hot gases, and will not have burn marks.
Table 1 shows a variety of methods of firing bricks. All have their advantages and disadvantages. Our typical down-time brickmaker will probably only have experience with clamps, scoves, Scotch kilns, and Dutch kilns. The brick clamp is by far the oldest and most rudimentary method of firing bricks. When “scoved” (that is, plastered on the outside for greater efficiency), they become scove clamps or kilns. If the clamp is enclosed within four permanent walls, it becomes a rectangular Scotch kiln. Dutch kilns are simple up-draft kilns and are a development of the kilns used by the Greeks and Romans. All of these “kilns,” plus down-draft kilns, are what are called intermittent kilns, where fires are set and then die. In continuous kilns the fire never goes out. Either the fire is continually moving, or the brick is moved through a fire zone.
Down-time brickmakers have not adopted continuous kilns because their intermittent kilns have proved entirely satisfactory. In addition to their ability to do the job, they are easy and cheap to construct. The problems facing the down-time brickmaker are related to the volume of bricks being produced and the cost of transport. Firstly, the average brickmaker will not be producing sufficient volume of bricks to justify pursuing improved kiln designs. Then there is the effective limit of about four hundred bricks per wagon which, when combined with the poor excuses for rural roads, means that the cost of transporting bricks more than four miles by road renders them an uneconomic option for construction. It follows that brickmakers will not invest in fancy permanent kilns when they may have to abandon them every time they move to stay close to their market. Add the seasonal nature of brickmaking and you begin see why brickmakers might choose not to invest in expensive structures that will sit idle for much of the year.
If the brickmaker isn’t already doing it, the first thing up-timers might recommend is that a roof be constructed to protect the kiln from the weather. Drafts or rain hitting the kiln exterior will cool down the kiln, increasing the amount of fuel required to fire the bricks. A roof will also protect firewood placed on top of the kiln where it can be warmed and dried.
The choice of fuel for firing will be the first major contribution up-timers can make. Down-timers are currently using charcoal, wood, grain husks, sawdust, and even, especially in the case of the Dutch, peat. In Thuringia, at least, there is a problem with the firewood supply. There is no way brickmakers would be able to fire significant volumes of bricks year round using wood or charcoal. There just isn’t enough unallocated wood available to satisfy the demand when every thousand bricks requires something like a cord of dry wood (the equivalent of about half a ton of bituminous coal). Up-timers can immediately introduce the idea of using coal and gas for firing, and in the longer term, they can introduce oil firing.
The next advance will be the introduction of new, more efficient kiln designs. The first new design is likely to be a variant of a down-draft kiln design, where the hot air and gases are pulled down and around the mass of bricks. This means the hot gases are in contact with the bricks for longer than in an up-draft kiln. Clamps and up-draft kilns are usually hottest at the bottom, meaning those bricks set lowest are exposed to higher temperatures than higher set bricks. This results in lower bricks being over fired while higher bricks can be under fired. The down-draft kiln reduces the differential with most bricks exposed to the same temperature, ensuring a better average quality product and lower failure rates. Down-draft kilns are also inherently more efficient than up-draft kilns and so it is easier to develop a more efficient down-draft kiln than it is to improve the efficiency of up-draft kilns.
As the USE starts producing more and more bricks, one of the most important objectives will be reducing the amount of fuel required to fire the bricks. Table 1 shows a selection of kilns and it also gives a range of coal consumption to fire one thousand bricks. Our levels of efficiency are likely to be low, as it has taken up-time brickmakers years to develop the materials and technology to achieve the levels of efficiency they now experience. The USE will be forced to introduce continuous kilns if they want to economically produce brick in high volumes.
All continuous kilns gain most of their efficiency gains by making the maximum use of the heat generated. They function a little like heat exchanges. Green brick enters the system through the exhaust from the fire. By careful management of the design of the kiln the green brick meets the fire zone being completely dried out and heated to over eight hundred degrees centigrade. With a desired firing temperature of about a thousand to twelve hundred degrees centigrade the brickmaker only needs to consume enough fuel to boost the temperature of these bricks another two to four hundred degrees. This results in a considerable reduction in fuel consumption compared with intermittent kilns. At the other end of the kiln, cold air is drawn in through the hot bricks. This cools the bricks at the same time it heats the air. This means that fresh hot air is fed into the fire. The fire doesn’t have to heat the air, so less fuel is consumed.
4a) Kilns: Some advantages and disadvantages
i & ii) Clamps, Scoves, and Scotch kilns
These kilns are easy to build and require little investment to construct. Having (except for the Scotch kiln) no permanent structure, they can be built close to the supply of clay and fuel, so that transport costs are kept to a minimum. In a time of war the brickmaker can afford to abandon his brick yard, which will be little more than a bit of level ground with a heap of dug clay. Clamps and scoves can be made to any desired size, from a few thousand bricks right up to a million. They are ideal for small teams, as once lit they require little attention, because all the fuel was included before the fire was started.
Of course there are problems with these kilns. The Colonial Williamsburg website talks about as many as half of the bricks being fired in their clamp being either over or under fired. This is probably the extreme failure rate, but it does point out a major problem with clamps. Not only are they among the most inefficient methods of firing bricks, they also produce the worst quality brick. There is little that can be done to change this state of affairs, as the brickmaker has no control over the firing once it has started.
This style of kiln is known to down-timers. It is only suitable for making low quality bricks because the average firing temperature only passes seven hundred degrees centigrade in the better constructed versions. It should only be used for brickmaking when you need bricks in a hurry and aren’t too concerned with the quality of the bricks.
iii) Up-draft kilns
Up-draft kilns are old technology. We know they were used by the Greeks and Romans, and currently (1630s) they are being used by the Dutch (hence Dutch kilns) to make bricks. They are a simple permanent design that has a much lower capacity than the clamp, but offers some control over the firing process. Because heat is introduced to the bottom and passes through the brick mass to an opening, these kilns are usually hottest at the bottom. This uneven distribution of heat is responsible for most of the twelve percent of bricks that are over or under fired. The average firing temperature of an up-draft kiln is about nine hundred degrees centigrade. However, at its hottest point, closest to the fire, it can be hot enough to fire firebricks. By carefully choosing what bricks to put where, a skilled brickmaker can take advantage of the peculiarities of the up-draft kiln to produce a range of bricks.
The up-draft and the Scotch kiln are probably the most common designs in use in the seventeenth century. This design has been used for centuries to fire ceramics, meaning that they can be used for something other than bricks. There are a number of improvements possible for the up-draft design, such as multiple chamber designs, but they are mainly targeted at the ceramics market rather than the manufacture of bricks.
iv) Down-Draft Kilns
The previous kiln designs all tend to lack permanent roofs. The up-draft kiln is dependent on its roof to function properly. The roof curve causes the hot gases to curl back into the mass being fired. The hot gases are then drawn through the mass being fired, escaping to the chimney through flues in the floor of the kiln. Because the fire is not in direct contact with the mass, and the air mixes as it curls back from the roof, there are few over- or under-fired bricks produced in a down-draft kiln. Down-draft kilns are typically more expensive to construct than up-draft kilns because the roof needs to be carefully constructed with a curve. They make up for the increased cost by being intrinsically a more efficient design which is easier to fire to high temperatures. A good down-draft kiln can be fired to over twelve hundred degrees centigrade, with some capable of firing at porcelain temperatures (thirteen hundred degrees centigrade and higher). Because the down-draft kiln can be fired to higher temperatures, the quality of the bricks produced will be higher than in up-draft kilns. They can also be used to fire any clay-based product.
A close cousin to the down-draft kiln is the cross-draft kiln. Instead of having flues under the floor, the flue opening is opposite the fire, and placed low in the wall. Again the hot gases circulate in the chamber, and are then drawn across the bricks and through the flue. Although not as efficient as the down-draft kiln, the cross-draft is cheaper to construct, and is still more efficient than the up-draft kiln. It is also an easier design in which to introduce shuttles (more on shuttles later).
The down-draft kiln is likely to be new to down-timers. However, it is a common design for modern potters. If there are any up-time potters they will know about down- and cross-draft kilns, and probably have reference material on how to design and build them.
v) Bull’s Trench
The Bull’s Trench kiln is a variant of the Hoffmann kiln (more on the Hoffmann kiln later). Designed by British engineer, W. Bull, in about 1887, this archless version of the Hoffmann kiln is widely used in Pakistan, India, Bangladesh and Myanmar, but is little known elsewhere. Its greatest advantage is its low cost of construction and comparatively low energy consumption compared to the local clamps and intermittent designs. The secret of the Bull’s Trench lies in the fact that, instead of having a massive structure, the kiln is dug into the ground.
The Bull’s Trench will be unknown to down-timers, and probably unknown to most if not all up-timers. The only people who might know about the Bull’s Trench design are likely to be people who have worked in Pakistan, India, Bangladesh or Myanmar. The Bull’s Trench kiln is unlikely to be used in Germany. The Bull’s Trench design requires dry ground, otherwise you expend heat energy drying out the ground. Also, any rainfall can flood the trench.
vi) Tunnel Kiln
The tunnel kiln is a special version of the continuous kiln arrangement. Whereas in the Hoffmann design the fire moves while the bricks remain stationary, in the tunnel kiln the fire zone remains stationary while the bricks move through the fire zone. This offers a major advantage over the Hoffmann design. Instead of having multiple chambers that heat up and cool down as the fire passes through them, the tunnel kiln has only one area that is continually exposed to the same temperature. This means no extreme changes of temperature anywhere in the kiln. There are savings, as only the fire zone has to be faced with expensive firebricks capable of withstanding the high temperatures of firing. There is a downside, and that is the bricks have to be carried on trolleys. Trolleys are a useful method of moving bricks and offer savings in handling costs. However, the person running a tunnel kiln needs sufficient trolleys to completely fill their tunnel (anything from one hundred to three hundred feet of tunnel), and have some outside the kiln being loaded or unloaded.
Although most modern tunnel kilns are built on the flat, and usually within a much larger structure that shelters the kiln and the loading and unloading areas, a low-tech version of the kiln is possible. By having the structure built on a slope, the tunnel itself acts as a chimney, causing a natural draft to pass up through the tunnel. Meanwhile, gravity can be used to feed trolleys of green bricks down the kiln.
The tunnel kiln will be unknown to down-timers. There should be some reference to the tunnel kiln in most good encyclopedias, probably enough for people to know what the concept involves. Additionally, up-timers are more likely to know of the tunnel kiln than the Hoffmann kiln. This is because the tunnel kiln is a more popular kiln design in America than in Europe. The cheap energy in America made it more desirable for manufacturers than the more efficient but more labor intensive Hoffmann design. The tunnel kiln will be considered for use down-time; however, there is still the problem with all those trolleys. Also, labor is still cheap in Germany, while energy is expensive. The reasons why the Hoffmann design dominated European brickmaking will continue to hold.
vii) Hoffmann Kiln
The first continuous kiln was invented in Germany in 1857 by F. E. Hoffmann. The Hoffmann kiln is basically a ring of down- or cross-draft kilns. It has all the advantages of the down-draft kiln with the added benefit of using waste heat to dry and heat the bricks before they are fired. Fuel consumption in a Hoffmann kiln can be half that of a normal intermittent kiln for the same mass of brick. The problem is the size of the structure. A Hoffmann kiln tends to be a massive structure that absorbs a lot of heat as the firing zone moves forward through the cold kiln. This is compensated by the fact that some of the residual heat in the kiln and fired bricks is used to preheat the air for combustion.
Any books on kilns, even articles in encyclopedias, are likely to talk about the Hoffmann kiln. For this reason it is reasonable to assume that the Hoffmann kiln will be known to up-timers. In fact, unless there is good reason to suppose someone in the Ring of Fire area has information for the Vertical Shaft Brick Kiln, the Hoffmann design or some variant of it (say four chambers connected in a square pattern) is the most likely continuous kiln to be built.
viii) The Vertical Shaft Brick Kiln (VSBK)
The VSBK is quite simply a vertical tunnel kiln. It has the advantage of a stationary fire zone, the advantage of a vertical chimney creating a natural draft from the bottom to the top, and the advantage of counter flow heat exchanging. Toss in the facts that for its capacity it has a very small footprint, fuel consumption is about half the next best design, and emissions are lower than most other kilns, and you have the ideal kiln for making bricks in 1630s Germany. If we then add that it is a design uniquely suited to using the German wet coal, it becomes almost a must have design.
Developed in China during the Cultural Revolution (1960s), there are thousands of VSBKs in rural China. In more recent years Chinese engineers have been introducing the technology to their neighbors. Since the start of the 1990s VSBKs have been built with Chinese assistance in Pakistan, India, Bangladesh, Nepal, Vietnam, and probably other countries. The VSBK is cheap and easy to build, and can be built in six weeks using mostly down-time materials and labor. The single up-time contribution, assuming down-timers can’t make sufficiently strong ones, is a screw jack per shaft capable of lifting a five-meter stack of bricks (up to thirty tons). Even the amount of information needed to build the VSBK is small, with a group in Nicaragua building a single shaft VSBK based only on information gained from the Internet. Production in the kiln is up to about seven thousand bricks per day per shaft, and VSBKs with as many as six shafts have been built. They can’t compete on volume with the bigger Bull’s Trench and Hoffmann kilns, however they are significantly more economical to run, and they have significantly fewer failures. Because the fuel is added to the green bricks the fuel undergoes the same drying and warming process as the bricks. This is what makes the VSBK so suitable for firing using the wet coal to be found in Germany. Other kiln designs introduce the fuel directly into the fire zone. They need dry fuel, otherwise heat will be wasted to dry and heat the coal. Meanwhile, the VSBK uses waste heat to achieve the same result. No other kiln design is going to be as economical to fire using the wet coals.
There is of course a downside to the VSBK. As they have to be able to withstand being stacked five meters high in the firing shaft, good quality green bricks are a necessity. Green bricks have to be lifted (or wheeled up a ramp) to the top of the tower, and then carefully placed in the top of the stack. The big problem is the twenty-four hours a day, seven days a week running of the kiln. Whereas all the other kilns need little more than monitoring of the fire during the night hours, the VSBK needs to have batches loaded and unloaded at regular intervals. Ideally, batches of bricks are removed from the bottom of the stack every sixty to ninety minutes, although in practice three to four batches are often removed at one time. This means, that for continuous operation, a source of light for the night shift is required. In a multishaft VSBK the workers can be busy right through their shift unloading from the bottom and reloading at the top.
The number of batches and the time of unloading are decided by the fire master, who uses his experience to judge when bricks are ready by the color and position of the firing zone in the shaft. This means that the fire master needs to be a skilled individual for the production of good quality bricks. The cost of poor skills in the few areas needing them can be seen in the failure rates in different countries. China has about two percent failures, India, where Chinese engineers ensured the staff was properly trained, have about a five-percent failure rate. Pakistan, where the Chinese engineers left without giving proper training, the failure rate is about twelve percent, about the same as that of the much cheaper to build Bull’s Trench.
It is unlikely that anyone in the Ring of Fire has ever worked on a VSBK. It is also doubtful that anyone in the area has anything, photographs, drawings, or even downloads from the Internet, on the VSBK. There is a limited possibility that someone might have seen a VSBK in operation in Asia or Nicaragua. However, as the design concept is so simple, I think that if someone has ever walked over a VSBK and seen one in operation, that person will have sufficient information for skilled up- and down-timers to develop a design.
4b) Shuttles and kiln productivity
Having decided on the design of our kiln, what else can we do to improve productivity? One area is reducing turn-around time. In most kilns bricks are hand set, and also removed by hand. This means that both the kiln chamber and the bricks must be sufficiently cool for humans to work. However, economics rears its ugly head once again. Every degree the kiln cools is another degree the brickmaker has to expend fuel to recover. This means workers will be sent into the chamber to unload as soon as the conditions are bearable. Then we have brick setters working in confined spaces with poor illumination as they race to empty the fired batch and reload the chamber as quickly as possible. There is a way around the problem however. We can use shuttles or trolleys. A shuttle is a cart with a metal frame and metal wheels, and decked with brick, often firebrick. Green brick is set on a waiting shuttle outside the kiln. When a batch of bricks is finished being fired and cooled to a safe temperature (such that it won’t crack when exposed directly to normal air temperature) it is simply wheeled out, and the new batch wheeled back in its place. Not only can setting and unloading be done in the open and relative cool, but the kiln is out of service for a much shorter period of time and doesn’t lose as much heat. By using shuttles, handling of green and fired brick and the associated costs can be reduced considerably.
Shuttles can not be used in clamps, scoves, Scotch kilns or most up-draft kilns. Either the uncontrolled nature of the firing or the fact that the fire must pass up through the brick means they are not suitable techniques. Down-draft kilns (and their close cousin, the cross-draft kiln) can use shuttles. The Bull’s Trench kiln, because of the hole-in-the-ground nature of the design, is not suitable for using shuttles, besides, the open air nature of the loading and unloading removes many of the benefits available. The tunnel kiln, of course, needs shuttles to carry the bricks through the tunnel. The VSBK does actually have a place for the shuttle, although its shuttle doesn’t need to be decked with brick. The VSBK is unloaded by lifting the loading deck of a trolley up to the bottom of the stack of bricks using a screw jack. Once in contact with the stack of bricks the steel poles holding the mass of bricks are removed and the trolley is slowly lowered until the next specially arranged layer of bricks appears. The steel bars are once again threaded through the gaps in the rows of bricks and the trolley is slowly lowered until the poles once again rest on a pair of heavy steel girders on either edge of the shaft. With the weight of the shaft of bricks taken on the girders, the trolley is lowered to the ground and wheeled out.
Unless they are prepared to spend a lot of time and resources to develop the knowledge base, up-timers will need down-timers to provide the detailed knowledge of clay and firing if they want bricks. Up-timer technology and knowledge would help down-timers increase brick production, but a simple increase in demand for brick will be sufficient motivation for down-timers to develop improvements. Put bluntly, we need down-timers; they don’t need us.
Who in Grantville will have the knowledge that will help the great step forward in brick production? We know there are going to be people in the Ring of Fire area who are potters or ceramic artists. For a start, on the grid of up-timers, we have Garth Freeman and his wife Melba Sue. They have ” . . . a very elaborate ceramics and pottery setup.” This is sufficient to suggest they have at least one kiln, and some clay preparation machinery, such as a pug mill. This being West Virginia, they probably built both the kiln and the pug mill themselves. This suggests that they have books and magazines describing kilns, ceramics, and machines for preparing clay. Also, Grantville is set in an area of West Virginia were ceramics were manufactured. There is good clay inside the Ring of Fire, maybe even China clay (suitable for porcelain), for this reason Garth and Melba Sue won’t be the only people in the Ring of Fire area who are into ceramics.
The small hobby ceramic artists and potters can get the industry started, but when it comes to the heavy extrusion machinery, specialist books will be desirable. This brings us to the late George Bowers. George died in about 1945, however his legacy survived through his daughters until June 2000, when his accumulated collection was auctioned off. George is important to us because he made his fortune from the manufacture of ceramics (mainly bathroom porcelain). The earths around Grantville can be assumed to be similar to those around Mannington where George made his fortune. It is reasonable to assume that, although George’s massive collection of books (some twenty thousand volumes on a range of topics) does not exist in Grantville, someone in Grantville is likely to have some of the same turn of the century ceramics books and treatise on brickmaking that George might have had.
Overall the prospects for the Grantville brickmaker are favorable. If they can find a down-time master brickmaker to do the hard work of selecting the clay and judging the firing of the bricks, there is sufficient knowledge within Grantville to rapidly increase the supply of bricks. Even without the advantage of reference material, the brick extruder should be developed. Knowing bricks were made with an extruder will be sufficient to set inventive minds on the right track. The idea of using shuttles should be obvious, as nobody really likes double handling of goods when they don’t have to. It is in the design of kilns where Grantville will have to be lucky. With all those chambers that have to be built to take high temperatures, the full Hoffmann kiln is going to be expensive to build. Either someone is going to have to suggest making a smaller version of the Hoffmann (say, four chambers set in a square), or someone is going to have to know about the tunnel kiln, or better yet the VSBK. The Tunnel kiln is likely to be thought of, but it is going to be expensive to make all those trolleys. Also, it is less efficient than an equivalent capacity Hoffmann kiln. I still like the VSBK because it is cheap to build and run; also it can easily be constructed with down-time resources. All it needs is someone to have seen one in operation, or to have done a little research on them for some reason.
Burnt:Bricks are burnt or fired in a kiln. It doesn’t mean they have anything wrong with them.
Clot: A clot of clay is a tempered lump of clay that is thrown into a brick mold. Ideally a thrown clot of clay will just fill the mold.
Continuous kiln: A kiln where the fire is kept going. The fire may move through the kiln complex, or remain stationary letting the bricks pass through the fire zone, but the fire never goes out.
Earth(s): The technical term used to refer to soils as opposed to rocks.
Extruded brick: When clay is forced through the end of a pug mill in the form of a continuous block which is then cut into small slices using wire cutters, those slices are called extruded brick. The size and shape of the nozzle defines the shape of the block extruded and thus the shape of the brick.
Fired: Bricks are fired, as in, exposed to the heat of the fire. See also “burnt.”
Fire zone:Wherever the fire is burning in a continuous kiln. The fire zone can move from chamber to chamber or stay in one place.
Frog: Another name for the “kick.”
Green brick: Brick that has been molded to shape, but has not yet been fired. With the addition of a little water, and a little hard work, plasticity can be restored to the clay in a green brick.
Grog: Take a broken previously fired brick or other piece of ceramic, grind it up, and use it as a filler. Grog does not regain plasticity and will not undergo chemical change when re-fired. Grog is useful for padding out supplies of expensive clay, or when the available clay needs certain additives.
Hack: To closely stack drying bricks edge on top of edge. Usually the stacks are arranged with about a finger-width distance between them. They are then left to dry a little longer. A hacking ground is an area where bricks are hacked (stacked).
Intermittent kilns: Kilns where the fire is lit anew for the firing of each separate batch.
Kick: The insert in a brick mold that causes a depression or hollow to be formed in the top of a brick. Often the brickmaker will have his name, or his company’s name set into the kick. The kick helps force thrown clay into the bottom corners of the mold.
Mellowed: Made pliable and plastic, usually by tempering.
Pallet: A board sufficiently large enough to hold a single brick with space left over. For a 9″ x 4.5″ x 3″ the pallet would be something like 12″ x 8″.
Plastic: This describes the condition of properly tempered clay. It can be easily worked (shaped and reshaped). If left alone it will hold its shape sufficiently long to be fired.
Setting: The careful stacking of bricks within a kiln so that they will be properly fired.
Strike: Basically a piece of wood with a straight edge that is used to smooth off any excess clay from the top of the mold
Tempering: The art of making clay plastic and pliable by breaking it down so that water is in contact with every molecule of clay.
Daniel Rhodes (1968). Kilns: Design, Construction, and Operations.
As the title suggests, a book telling the ceramics worker the history of kilns, how to design a kiln, build it, and run it. The book also provides plans for a selection of small ceramics kiln. Almost a must have for the Grantville home ceramic artist.
Edward Dobson (1903) (11th ed.). A Rudimentary Treatise on the Manufacture of Bricks and Tiles Containing an Outline of the Principles of Brickmaking.
This book examines brickmaking in England. It provides good descriptions of the process and also includes illustrations of many of machines for tempering clay and extruding bricks and sewer pipe.
Charles Thomas Davis (1895) (3rd ed.). A Practical Treatise on the Manufacture of Bricks, Tiles, and Terra Cotta:
This book describes the industry in America. It fully covers brickmaking, the machines that can be used, and many of the kilns.
Alfred B. Searle (1924) (2nd ed.). Refractory Materials: Their Manufacture and Uses.
Searle examines the refractory industry in England and Wales. There is a lot of information on where to find the various fireclays, and how to mix them for desired refractories.
Colonial Williamsburg Brickyard: Eighteenth Century Brickmaking in the USA.
Vertical Shaft Brick Kilns (VSBK)
The Vertical Shaft Brick Kiln (VSBK) In Nicaragua
An article on a group constructing a VSBK in Nicaragua using only the information they can find on the internet and in a few pamphlets. There are a few good photographs of the VSBK being built.
Bull’s Trench Kiln
Lots of other interesting documents on brickmaking.
A history of Isle of Wight Brickmaking