A furnace is used for many different types of work, including blacksmithing, metal smelting and even glass production. A furnace is extremely simple to make. Before you begin, however, make sure you design the furnace precisely.
The type of furnace described here is used widely by Asian and the Middle Eastern blacksmiths. The design of this furnace can be altered to whatever shape or size you want but the process for making it will always be the same regardless of the size. The furnace described here is a coal fuelled furnace. These are the simplest and quickest to build and design. It is 30 inches long, 10 inches wide, and around 12 inches high and is an open furnace and thus has no cover or lid.
Acquire around 20 firebricks. Purchase more or less bricks depending on how large you want your furnace to be, but 20 bricks should be more than sufficient for a small furnace.
Lay down the foundations of the furnace. Make a base out of firebricks about 30 inches long and 10 inches wide. Construct the base on a concrete or stone floor as this is the easiest and safest place to have the furnace.
Build the walls of the furnace. On either side of the base construct two walls with the left over bricks. These walls should be around 12 inches in height and 30 inches long. The coal will be stacked between these two walls and this is where the fire will be. There is no need to build a roof onto the furnace.
Drill a hole in one of the walls. This hole should be about one inch in diameter and can be located anywhere along the wall, preferably as close to the base as possible. This hole is what will allow air to be pumped into the furnace thus regulating the heat. This is one of the most important features of the furnace as it will allow you to increase the heat of the fire, even to the point of melting steel.
Attach a bellows. Insert a bellows or anything that pumps air into the hole. The air flow will increase the heat of the fire. Make sure that the nozzle of the bellows is iron or steel so that it doesn't melt easily. If it is not, then you can attach the nozzle to an iron pipe that will lead into the fire.
If you do not have a bellows or a fan, you can use a hair dryer to pump air into the furnace. Simply attach some iron piping so that the air pumps into the furnace. This works extremely well and very high temperatures can be reached by this method.
Investing in a good furnace thermometer is advisable so you know exactly what heat range the furnace is reaching. These can be purchased at hardware stores or online for less than $100.
A furnace can reach extremely high temperatures. Do not place it on or near any wooden constructions.