Re: [hobbicast] Re: Refractory blanket how-to for welding forges.
- Hey there, Folks-Just to chime in. Mike's idea, about casting a shelf into the furnace bottom, is where having a wax candle as a central core really is a really great idea. It's very easy to carve the candle to provide the size and shape shelf desired in the finished in the finished casting.And now I leave the tracks behind & propose something that may or may not be helpful, just because it occurred to me. So please comment, oh you who know better: If I were to undertake such a project, would I be wise to score the surface of the candle that makes the shelf so as to produce a set of raised lines on which to lay my workpiece?This would greatly reduce the area of contact between the workpiece and the shelf. I guess the crux of the matter is whether or not this will let the piece be more evenly heated, or cause very small irregularities instead. And would in irregular surface create unnecessary turbulence, to the point where it might effect performance? For that matter, I presume that the incised lines should in line with the length of the tube. What about perpendicular to it? Any comments?Thanks. (I guess I had a bit to put out for a lurker.)Mark FeldmannPacifist ...with occasional lapses.
On Sep 1, 2013, at 3:04 PM, michael.a.porter@... wrote:
But the thoughtful reply would be to suggest you use another building scheme altogether. A lot of guys build their casting furnaces by investing in a bag of their favorite castable refractory (may I suggest Kastolite 3000, or if you're planning to use a really good burner, even the new hotter rated products from its manufacturer HINT, HINT). Build a mold from two different concrete pour tubes (available at large hardware stores), or one pour tube and a large candle. Trap the parts on a board, and make your refractory hot-face. Place whatever structure you desire for an end wall in the bottom of your forge shell, center the finished refractory tube, and pour in a mixture of the same refractory mixed down 50/50 with Perlite as your insulating layer; this is an abbreviated list of instructions. You know, for instance, that provision must be made for the flame passage from burner port through various refractory layers, for instance. And, you may also want to build a flat side into the tube form before pouring, so as to have a built in bottom shelf.
Michael A. Porter
5101 S. Mead Street, Seattle, WA 98118
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Recently someone on one of the casting groups commented that there was no such thing as a cement that can have all of the water baked out of it; I believe this was done in defense of the use of Portland cement in a homemade castable refractory. The official answer would be that the chemically locked portion of water in lime based cement cannot be baked out, but the chemically locked portion of water in refractory cements can be. I have always accepted this official version as the only reality...in the past. The limiting factors on "baking out all the water" are that the refractory must be taken to yellow heat in the first place, and that over time, water content can recollect in refractory if you're not careful to seal the refractory surfaces against water vapor in ambient air.
However, truth of any kind is seldom found effortlessly; including technical "truths". I suspect that a healthy debate, with both sides airing their views, might adjust what the majority of us accept as practical reality--to our mutual profit; this could be important for people wanting to make insulating refractories as secondary layers. Homemade refractory as an insulating secondary layer might be quite forgiving of official standards; standards useful for hot-face layers may constitute a waste of money in secondary refractory layers.
While casually dismissing the Portland cement idea, I have noted both resentment, and a strong hint of "I'll match your official facts with personal experience" in passing (heated) comments from the other side of this issue. Isn't it time they had a FAIR hearing; something open minded, maybe?