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I Forge Iron

Homemade Furnace Liner(also can be used in a fire pot)

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4 gal. part fireclay
4 gal. grog
4 gal. silica sand
3 quarts of borax powder

Fireclay can be gotten at most fireplace stores, and lumber yards. Grog is crushed fire bricks. This is a key ingrediant, to eleminate shrinking, and cracking.

Mix thoroughly while dry.

Water should be sprinkled over the dry mixed ingredients while stirring, until mixed completely to a consistency that can be made into a ball in your fist that can be broken cleanly in two. When it is to that point, you should cover with plastic over night.

It will keep indefinatly in a sealed bucket.


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  • 1 year later...
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are there substitutes to firclay which work as well?

cat litter (bentonite clay)
but it needs to be ground

Can I make the grog myself If I cant find It?Or is there a sub I can find @ home depot.

grog is a clayceramic that has already been fired, if you have a local potter, his brokenexploded pots would be ideal, again needs to be broken up to a suitable granular dimension

other insulative additives
Sawdust (the coarser the better, chainsaw particles are ideal) turns to little charcoalized insulative pockets

additional insulative barriers will make the forgekilnfurnace more effcient, a box in a box or barrel in a barrel with rock wool, Vermiculite or other air trapping fireproof material with a low thermal mass

the more thermal mass the forgekilnfurnace has the more energy it will take to get it up to a given temperature, but the better it will hold that temperature (principle behind a masonry heater) but unless you plan on leaving it on for days at a time, its better to have a low thermal mass with a high insulative value
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What's the purpose of the borax powder? Doesn't it tend to melt under high heat?

good question,

lets look into the crystal ball....

patent 4233078 (pdf)

Sodium borate, preferably a hydrated sodium borate and most preferably borax (the decahydrate), is added to a graphitic refractory composition suitable for forming a refractory monolith, for example a plastic or a ramming mix, in order to reduce the loss of plasticity of the mix during storage, to inhibit fungi formation in the mix during storage, and to improve the intermediate temperature strength of the composition

Dehydration Behaviour of Borax Pentahydrate to Anhydrous Borax by Multi-Stage Heating in a Fluidized Bed Calcinator (PDF)

Anhydrous Borax (BA) is an industrial product produced from borax decahydrate and borax pentahydrate. It is used as a flux material in metallury, and in the production of high quality glasses, ceramic materials and enamels.

it goes on to say that the amorphous form we know as a metallugical flux, is actually just one of several states, there are also 3 different crystalline forms with melting points of 1015K (1367F), 993K (1327F) 936K (1225F)

anhydrous of course means without water, so its likely we arent employing BA specifically

The low melting point of BA causes very dangerous corrosion problems which can only be prevented by forming a layer of solid calcined borax on the refractory material of the furnace.

best guess its added for multiple reasons but one of the most likely is to form calcinated borax on the refractory
still researching ;)
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Re the use of sawdust am I getting this right? The idea is that the sawdust actually burns and thereby leaves small empty pockets or pockets containing charcoal which due to it's lack of mass increases the thermal insulation properties of the monolith. Please let me know if I am wrong! I am looking for a good insulator for round my cupola.

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