How Are Bricks Made? Shaping Clay So Fire Can Set It

Clay consists of extremely fine plate-like mineral particles that slide over each other when wet, which is what makes it plastic and shapeable, and that hold water between them. Drying removes the water between particles and leaves a fragile solid that will turn back to mud. Firing changes the minerals themselves. Below about five hundred degrees the chemically bound water is driven out of the clay structure, an irreversible change after which the material can no longer be rehydrated into clay. Above roughly nine hundred degrees the particles begin to sinter, fusing at their contact points, and fluxes present in the clay, particularly iron and alkali compounds, start to form a small amount of glass that binds everything together. Continuing to higher temperatures produces more glass, less porosity and a stronger, denser and less absorbent brick, up to the point where the whole mass would slump. The colour comes largely from iron content and from whether the kiln atmosphere has plenty of oxygen, which gives red, or is starved of it, which gives the blue-greys of reduction firing.

Modern production is continuous and the stages map closely onto what was done by hand for millennia:

• •Winning and preparation, digging the clay and blending different deposits to a consistent composition, then grinding and screening it
• •Forming, by one of three main methods: soft mud, in which very wet clay is thrown into sanded moulds and which gives the irregular character of handmade bricks; extrusion, in which stiffer clay is forced through a die as a continuous column and cut by wires, giving the crisp wirecut bricks that dominate; and dry pressing, using nearly dry clay compacted under high pressure
• •Drying, which must be slow and controlled, since water leaving too fast cracks the brick, and which typically takes a day or more in heated tunnels
• •Firing, at around a thousand to twelve hundred degrees depending on the clay, with a carefully controlled rise and fall since thermal shock cracks the product
• •Cooling, which is as slow as heating and takes a comparable time
• •Sorting and packing, with bricks graded by appearance and performance
• •The kiln that made industrial production possible is the Hoffmann continuous kiln of 1858, a ring of chambers in which the fire moves around the ring while bricks are loaded and unloaded, so the heat leaving one chamber preheats the next and almost nothing is wasted

Brick dimensions have been remarkably stable and the reason is the human hand: a brick must be liftable and placeable in one hand, which caps it at around four kilograms and sets the length at roughly double the width plus a mortar joint, so that bricks can be laid both lengthways and crossways in the same course and still line up. That proportion is what makes bonding possible, meaning the patterns in which bricks overlap so that vertical joints do not align and a load spreads through the wall rather than running down a single line of mortar. English bond alternates courses of headers and stretchers, Flemish bond alternates them within each course, and stretcher bond, which uses only lengthways bricks, is now most common because modern walls are usually two separate leaves with a cavity, tied together with metal rather than with bricks. Bricks are also fired slightly larger than the finished dimension is expected to be, because clay shrinks during drying and firing by several percent, and the shrinkage must be calibrated to the specific clay.

Fired clay is strong in compression, poor in tension, durable for centuries, resistant to fire and to insects, and requires little maintenance, which explains its persistence. Its weaknesses are weight, poor thermal insulation compared with modern materials, and vulnerability to water penetration through mortar joints and to frost damage if water freezes inside a porous brick, which is why bricks are graded for frost resistance and why soluble salts in the clay can produce efflorescence, white deposits on the surface. The environmental problem is the firing, which requires sustained high temperatures and makes brick production a significant industrial energy user and carbon emitter, compounded in some regions by inefficient traditional kilns burning coal and biomass. Responses include better kiln design and heat recovery, blending in waste materials that burn out and reduce fuel needs, thinner and perforated formats that use less clay, unfired stabilised earth blocks for suitable applications, and reclaiming old bricks, which is straightforward where lime mortar was used and difficult where cement mortar bonded them too well to separate.

Firing drives chemically bound water out of clay and sinters the particles with a small amount of glass, an irreversible change after which the material never softens in water again. Bricks are formed in moulds, extruded and wirecut, or dry pressed, then dried slowly and fired at around a thousand to twelve hundred degrees. Their size is set by what one hand can lift, and the length being double the width allows the overlapping bonds that spread load. Firing energy is the main environmental cost.