For building ovens, forging metal, blowing glass or firing pottery, special heat resistant bricks are needed. These bricks are called firebricks. They are made from fire clay, which is actually a common form of soil often found along rivers and streams. The high percentage of silica and alumina in its composition makes the fire clay extremely heat resistant. Medium grade firebrick (suitable for ovens) withstands temperatures up to 1,482 degrees Celsius (2,700 degrees Fahrenheit). Firebricks are expensive, and you might need hundreds of bricks to build an oven. Making your own bricks is an interesting project and will save you money, but will take time to do right.
Buy fire clay online or in a shop, either from industrial sources or a local potter's supplier. Searching online, or in your phone book under "pottery supply" or "refractory materials supply." Fire clay can also be found in nature but can be difficult to identify without expert help. To find a local source, talk to the pottery/ceramics faculty at a local college or to working potter's in your area. However, many potters and ceramics makers consider the source of their clay to be a trade secret, and they might not want to share it. While digging your own clay might be satisfying, or even required for your purposes, the advantage of buying from a commercial source is that you can buy a mix that fits your specific temperature requirements, as the mineral composition of the clay determines how much heat it can withstand.
Mix fire clay with sand in a 1:1 ratio. Mortar needs to be thoroughly and evenly mixed. Clay from a natural source or purchased from a potters' supplier will probably be wet (about the consistency of play dough or modelling clay), and the consistency makes it difficult to stir. Mix it in a cement mixer or restaurant sized dough mixer. Also, clay from a potter's supplier may already have the sand added, so make sure to check with your source. You do not want double sand in the mix, as that will ruin the bricks. However, if you buy your clay from an industrial source, it will come dry and granulated, much like concrete mix, which makes it much easier to mix together either in a cement mixer or by hand.
When finished mixing the sand and clay together, add water until your mortar is the consistency of soft modelling clay. Be sure to mix completely so that there are no dry spots in the mix and the sand and clay are evenly distributed throughout.
Shape the bricks
Shaping bricks can be done in several ways. Probably the easiest is dumping out clay on a plastic sheet and then using a length of PVC pipe like a giant rolling pin to roll out the mixture to approximately 10 cm (4 inches) thick, or whatever thickness you desire. You can use other types of pipe, but PVC doesn't stick. Once you have the thickness right, take a trowel and cut the clay into desired sizes. 25 x 10 x 7.5 cm (10 x 4 x 3 inch) bricks are common, but you can make them any size you want. You can even hand form any shape you want. Plates and cups made from fire clay are often referred to as stoneware. When choosing a size for your bricks, remember, the thicker they are, the longer it will take them to dry. You can also make brick moulds of the desired size and use them like a biscuit cutter on the rolled out clay.
The drying bricks should be placed in the warmest, driest available space. In commercial firebrick factories, the bricks are "water dried" in special heated rooms to allow them to speed up the drying process, but that is not often possible in a DIY operation. Drying the bricks is the most time consuming part of the process, and the slower they dry, the less likely they are to crack or have their integrity compromised.
Bricks should be laid out on plastic sheeting or landscaping paper in rows, with at least a small gap between them. Then they should be covered with another sheet of plastic or paper, but don't try to seal edges. Having some holes in the sheets won't hurt anything. The idea is to slow the drying time so that the whole brick drys evenly.
Check on the bricks from time to time to see how they are coming along. When fully dry they will make a hollow sound, compared to the ones that are only surface dry. When you think that your bricks are completely dry, crack one from near the centre of the covered area open with a hammer or a masonry saw and make sure. If they are done, it is time to move on to the firing process.
There are two ways to fire bricks. Either fire them in a pottery kiln, or have it done for you. There will be local potters with kilns, and you should be able to talk one into firing your bricks for a fee. The other way is to build your oven, forge, or kiln with the dried bricks, then coating it with concrete to hold the outside together, and building a fire in the new oven. This method is appealing because it can be done at home. But it is difficult to control the temperature of the fire in your back garden; it needs to be quite hot, somewhere around 1400 degrees Celsius (2,700 degrees Fahrenheit), and extra high temperature applications may require even higher firing temperatures. With this in mind, if you are building anything other than an oven, you might want to have the bricks fired professionally.
In commercial brickmaking operations, the bricks are fired in enormous tunnel kilns, then allowed too cool in place. Once the tunnel cools enough for the bricks to be handled, they are unloaded and put in circulation. At home or at a potter's, you will not be able to fire too many at once, so you will have to do it over a period of time. It is not necessary to let bricks cool before removing them from kiln, but they cannot be handled without long handled tongs, and if you set a hot one on anything even remotely flammable before it is cool, you will start a fire. Be sure to get advice from an experienced potter before trying this yourself.
Once the bricks are fired and cool, they can be used to build anything you might need, just like ordinary bricks. You might need high heat mortar to hold them together, so consult someone with experience in the particular application you will be using the bricks for.
Be very careful when firing bricks and removing them from kiln because they may be very hot.