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 are 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 Stack 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 Stack 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.