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The Grantville Brickmaker's Primer

Written by Kerryn Offord

[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 pallet molding 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.

Figure 1. Grinding clay in a ring pit. (Dobson)

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 it's 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. ...