Furnace built from cheap YTONG bricks and Homemade Burners

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 ($3 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.

Burner

Such a burner can be made yourself with easy obtainable parts as long you can do some hard soldering. Hard solder for brass / copper is readily available at the local hardware shop. Building such a burner requires just basic tools. What you need: For optimization I made a burner from steel tubing where I used 25mm I.D. (28mm OD) for the burner nozzle. The picture below shows the nozzle pipe made from the 25mm steel tube. It is just made from central heating galvanized steel tubing. For the gas inlet a 15mm (OD) heating steel tube is used of which the end is soldered with a small disk op copper to close it. There is a small orifice (1mm diameter) in it to allow the gas flow into the nozzle.

(Cordless) drill and drill bits of 1,4,6 and 8mm
A burner (yes, a fully functional one) with which you can do hard soldering
Hard solder (usually silver based)
5mm thread tap + wrench
2 screws M5, 15 or 20 mm long
25mm ID steel (preferably stainless steel) tube about 20cm
10-12mm ID steel or brass tube about 50cm
8-12mm OD steel, brass or copper pipe about 12cm long
A burner head which fits on an existing propane burner handpiece
Gas leak detection spray. Some detergent in water with a brush can also be used as long as it bubbles.
Tank pressure (can be adjusted with regulator): about 1 bar

As the burner head screws are not separately available and if you don't have an abandoned burner head which fits onto the handpiece, you have to buy the cheapest burner head which fits onto your burner handpiece. You only need the screw and you make the feeder pipe to the nozzle yourself with the 15mm steel tube. You can use the brass coupling on the handpiece end for your new burner. This end has to be soldered to the thin 15mm tube which other end is to be closed by soldering a small brass or copper disk. Do not forget to put the nut on it for attachment to the burner handpiece. Drill a small (1mm or less) hole about 2cm from the top of the thin steel tube. When this is ready you have to check for leaks.
Do this test outdoors or in a well ventilated room ! Have a leak detection spray ready and apply it to all solder seams and screw threads. Connect the soldered steel tube on the handpiece, put your finger on the small hole and open the gas valves. No hissing or sizzling noise should be heard, no gas smell and no bubbles should appear. Apply even more leak spray. When this is leak-free, this part is ready for use.
The nozzle tube can be made by making two holes in it at about 5 and 10 cm from one (the air inlet) end of 4 mm and then tapping an M5 thread into it. If you use the burner without forced air, you have to drill air inlet holes of 10mm (about 4, each 90° apart) in the nozzle tube. You can make a slider by sawing off a 2cm long piece of the 25mm tube and sawing it through lengthwise and then bend outward in such a way that it just fits over the nozzle tube. This leaves an opeing of about 1cm but when closing the air inlet completely you can move it in such a way that it does not fit over a hole. Better is using a piece of slightly larger diameter pipe which just slides over the nozzle tube.

Schematic view of the burner burner

Detail of the copper tube to prevent flashback

The reason for the anti-flashback tube is to prevent that the flame 'strikes back' into the tube which results that most heat is generated inside the nozzle tube which gets red or orange hot. As a result the furnace gets far less heat. With screw (4) in the first picture, this tube can be moved axially.

Burner overview. Legend: 1. coupling to handpiece 2. Fixing screw for nozzle 25mm pipe 3. Fixing screw for anti flashback tube (not visible) 4. Air inlet regulation slider when using without forced air 5. clamp coupling of steel tube to copper extension of handpiece coupling	Burner attached to the handpiece
Coupling to the handpiece. This should be thoroughly checked for leaks !	Look into the flame end where the anti-flashback pipe is visible
End of the long small tube where the nozzle tube is attached. The orifice is in the red circle. Left the soldered copper disk.	The nozzle tube with the anti-flashback tube under it

Update 2014 Sep 6/7 Natural gas burner test

I realized that it should be possible to use natural gas for this small furnace to melt metals. To test this I made a new tube with a larger hole in it as the natural gas pressure is only 30mbar compared to 500mbar to 1 bar with propane. I drilled a hole of 2mm diameter rather than 0.7mm in the 10mm brass tube and used the same flare nozzle.
I used the same furnace, and succeeded to melt about 100g of copper in a few minutes. I checked the gas meter and counted about 0.2 liters per second which equals to a power of about 6kW.

Then I did a test with another small furnace made years ago from alumina bricks rated to 1700ºC, as YTONG cannot withstand temperatures higher than 1350ºC. I already tested this on high temperatures with propane and reached 1500ºC after 10 minutes. But now I tested this again with the new natural gas burner and the result was amazing: the same time 10 minutes to reach 1500ºC and finally 1550ºC after 15 minutes. To check the integrity of the type 'S' thermocouple, I was able to melt iron wire (it was binding wire made of 'soft' virtually pure iron, not steel which melts at 1539ºC). Of course a crucible with metal in it will take longer probably 15 minutes. In this test the furnace was empty. Gas consumption was about 0.2l / sec according to the gas meter which equals to a power of about 5-6kW.

Update 2012 Sep 29 Newly made burner with a test

I made a new version of this burner using an old roofmaker's 'roaring' burner with a 45° angled end. I cut off the angled end and soldered a copper cap onto it with hard solder. After soldering I blowed strongly into the tube while the solder seam is in a glass of water to check the soldering is gas tight.
Then I drilled a 0.75mm hole in it which is the orifice for the gas output. This is a tedious job as the drill bit should be put very deeply into the chuck and only 3mm left outside. During drilling no pressure should be applied on the machine as otherwise these very fragile bits break. I drilled holes in the nozzle tube and filed it to 12mm (the diameter of the long old burner tube) with a rattail file. On top I tapped an M4 thread to secure the nozzle tube to the inner tube. Finally I put the flashback arrester made from copper tube into the nozzle as described above.

Burner test 2012 Sep 29

Testing newly built propane burner

Natural gas heating furnace to 1500ºC 2014 Sep 7

Testing natural gas burner heating a furnace to 1500ºC

Pictures of making the burner