Grantville Gazette - Volume V (43 page)

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.

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.

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.