I made a hinged former out of 40mm x 8mm flat mild steel. The inside dimensions are 180 x 90. I rammed the not too wet mix then removed the green (un-fired) brick and let it dry in the sun for a day or so before firing it. The fired brick ended up measuring 175 x 88 x 38, the green brick would have been 180 x 90 x 40, so there has been a fair amount of shrinkage.
Because I fired it sitting in my furnace it hasn't fired evenly all over and probably could do with a bit more firing. As a quick check I put an Oxy-Acet flame to one side and it remained cool on the other side. I didn't try it for very long so can't really say how long it might take to get hot or to what degree.
The fired brick weighs about 850 grams. Antonio Ramos on whose recipe and ideas I based mine says that this size brick, should only weigh about 500 grams. I didn't go with his recipe because it also had other clays and I don't think I would have found those easily around where I live. Also I think my sawdust was laden with excess fine wood dust. Thanks Antonio!
I made up 5 Kg of fireclay/grog mix then added 50% of that volume of sawdust. The mix smells really nice :). To get 5 Kg I mixed 3.5 Kg fireclay, 1.5 Kg grog. That is a 70% fireclay 30% grog mix. I used as little water as I could to get a mix of good consistency without being too wet. Once mixed it should be let to dry out somewhat, to a similar consistency as correctly tempered moulding sand, before ramming it in place. I have no idea of what the absolute best ratio of fireclay and grog would be as I have seen various mixes mentioned and they apparently work so maybe there is a reasonably wide range that will work. Ideally the sawdust should not be too fine or contain wood dust, it should also not contain curly shavings. In B. Terry Aspin's book "Foundrywork for the Amateur" he mentions 50/50 fireclay/grog mix for lining a furnace.
Here is the grog, as I understand it grog is smashed ground up fire brick. This stuff is fairly fine, the largest particles are like coarse sand and down smaller to fines.
Here is the fireclay, it's rated to 1200 deg C, it's mostly real fine and dusty but there is some coarse stuff in it. I got this fireclay and the grog from a local maker of clay pavers and fire bricks. Claypave Now as soon as I can get around to it I want to make a new furnace using the same mix as in the insulating fire brick above.
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