Re: [hobbicast] Re: Refractory blanket how-to for welding forges.
- Dan recommended my book in the context of welding forge building; this brings up a subject I've been avoiding for years, but now it's time to "clear the air."
The standard method of ceramic fiber placement shown in Gas Burners for Forges, Furnaces, & Kilns, was to position two layers of ceramic blanket material against the inside of the forge shell, wrapping them in position so that they would be trapped in place (like any other arch; through weak material compression at their ends) while stiffening into a permanent shape (the material is well known for "taking a set" as it loses springiness through repeated heating). This method has advantages for the builder of a typical gas fired hobby forge (easy and economical), BUT not for the construction of a blacksmith's dedicated welding forge. Experts agree that upset welding can be dependably done at red heat, but nearly everyone feels the urge to weld at far higher temps. It isn't a how-to author's job to shake an expert's finger in the face of readers, but to point out the best path, and then to try to accommodate preferences.
An average would-be forge welder wants white heat. But, white-hot internal temperatures greatly accelerate shrinkage of ceramic fiber products. You're not going to find this problem touted in sales literature, but it's a hard fact of life; this leaves the builder of a general purpose gas fired forge fairly unmolested, until a fascination with forge welding develops; afterward, it usually leaves the victim switching to castable refractory and/or firebrick. But, there is a better way...
Dudley F. Giberson, Jr. (a pioneer of the American hot-glass movement, and author of A Glassblower's Companion) perfected the technique of cutting his ceramic fiber blanket into folded squares, and highly compressing them against one another to form arch (and tube) shapes, thus making a complete end-run around the problem of shrinkage in his glass working equipment way back during the early days of ceramic fiber availability to the public (it was originally developed for NASA in the seventies). No one has found a better method since. Rigidizer, and toughening coats work best when the surface beneath them is itself highly stable. The reason for adopting this method in construction of casting furnaces should also be obvious.
[Non-text portions of this message have been removed]
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?