Furnace built from cheap YTONG bricks

Ytong bricks are used for indoor walls in construction of buildings because of there excellent insulating quality. It is actually Aerated autoclaved concrete which consists mainly of calcium silicate which is formed in the autoclaving process during manufacture from silica and Ca(OH)₂. During earlier experiments in which I thought these bricks are nearly pure CaSO₄, I experienced its very good insulating capacity, so I built furnaces from this for melting metals. The material is very cheap ($7 for a 20x60x10cm block) and readily available at construction supply shops even those which cater to individuals.

As a fuel I use propane which is also easily to obtain at hardware shops here in the Netherlands in 5 and 12 kg tanks. You need a decent burner and a pressure regulator (not the 50mbar fixed low pressure heater or stove grade reducers) which reduces to at least 1 bar. Those are available at hardware or gas supply shops which are dealers of propane companies like Shell or Primagaz. The tanks should be purchased once and every next time you trade the empty tank for a full one and pay for the refill usually $20-$30 for a 12kg tank.
The furnace reaches easily 1100°C after seven minutes and 1300°C after 15 minutes even with natural drag (i.e. without an air fan). But a cheap camping store air mattress inflater with a homemade burner from steel pipes will increase temperature to 1450°C after 15 minutes at a lower gas consumption rate and a faster heating up. The gas sprinkling hole in the burner should be not larger than 1mm.
However at temperatures above 1350°C use another refractory as the YTONG bricks will be damaged quickly.

SAFETY issues

As you work with flammable propane gas and hot parts you must be aware of the risk of fire and burns !

• Perform these experiments outdoors or in a well-ventilated room. When using a fumehood be aware that the vent should vent off hot air and the vent and exhaust pipe can withstand air of over 100°C. Vents and pipes of PVC or other plastic are not recommended.
• Check that all gas valves, regulator and burner connections are not leaking. Apply a detergent solution with a brush on all couplings and no bubbles should appear. There should be no gas smell as well. After use ALWAYS close the main valve on the gas cylinder.
• NEVER let the furnace run unattended.
• When making your own burner TEST IT THOROUGHLY that there are NO LEAKS !
• Keep flammable stuff away from a running furnace.
• Have a fire extinguisher ready or at least a bucket of cold water in case of burns or fire.
• When pouring metals always use a face shield or at least goggles.
• Use heat resistant gloves when handling hot objects. Burns are not fun !!
• Use dark green goggles (available for about $10 at a welding or hardware shop) when looking at objects above 1350°C.

The benefits of this 'refractory' are (using a small furnace with a 50ml crucible with copper of cast iron):

• Cheap and easily obtainable
• Within 7 minutes to 1100°C, within 15 minutes to 1350°C in the crucible starting from a cold furnace
• Insulates very good: lid can be taken off with bare hands while other side is over 1000°C
• No castables or concrete required. Can be worked / carved easily.
• No pressurized air or oxygen needed, only a cheap $10 air mattress inflator from camping supply stores.

What to take care about:

• First run should be one slowly as the bricks contain moisture
• Lid and furnace body should be wrapped around with steel wire as they always break apart in two pieces (and not more!) after firing which can be held together
• A circular piece should be put within the furnace chamber. This can be made from the cutout when sawing the chamber with a circular saw.
• A flat piece of Ytong should be put under the furnace.
• This material can be used up to 1350°C. Above that temp it changes in a pudding like substance and after freezing and cooling again it deteriorates quickly.
• When not heating above this temp, it can be used 20 times or more.