I don't know what the rigidizer supplied with the Devil Forges consists of.
"Rigidizer" is a pretty vague term. Essentially, anything that makes the blanket more rigid after application than it was before application can be called a Rigidizer.
Rigidizer, when sold by the manufacturers of Refractory Ceramic Fibre blanket, is usually a colloidal suspension of finely-divided Silica in water. The particle size is selected to maximize the concentration and there are usually wetting agents and/or pH modifying additives in the composition to further increase the Silica concentration. Occasionally, rigidizers use Alumina instead of Silica.
I can claim no expertise, but my understanding of the action of the rigidizer is that it soaks in, coats the fibres and bonds them together where they touch. Once dry, there is an increase in rigidity, but the full effect is not achieved until the blanket/rigidizer has been heated to a temperature at which the Silica fuses the fibres together. Rigidizer is effective to the depth reached by the liquid when it is applied and can provide an increase in rigidity to the full depth of the blanket if applied in sufficient quantity. It does not provide a hard surface layer.
A coating such as Satanite will also rigidize the surface of the blanket. Satanite seems to be a trade name for a refractory mortar. Most refractory materials have properties that are largely dependent on the minerals from which they are made, so are dependent on the local geology. Most of these materials have relatively low value and they tend not to move very far from where they are mined/processed.
A few years ago, I spent a fair amount of time trying to come up with the cheapest reliably-repeatable way to make an effective Propane forge for knifemakers here in the UK. "Refractory Cement" is widely available here. It seems to comprise a certain amount of "sand", a certain amount of clay-like material and a sticky resin-like binder. Applied as it comes, it makes a lousy surface coating for blanket in a forge. Mixed with water, agitated vigorously, and allowed to settle for a minute or so, the sand sinks to the bottom so that the suspended clay and dissolved binder can be decanted off. Applying the decanted liquid liberally to blanket seems to carry the dissolved binder to full depth, while the clay-like material penetrates to perhaps 1/8" maximum, but mostly remains on the surface. It is a PITA to dry it adequately (though climate is clearly a factor here), and I found the best way with small forges was to put them in the oven on its lowest setting overnight. This scored me no points at all on the marital harmony scale: be warned. Failure to get it completely dry will result in the top layer drying as soon as the forge is fired, Trapped moisture will flash off to steam, bubbling the dried layer as a paper-thin skin. The skin will break and form little flakes of refractory, then the next layer will do exactly the same thing.
I also tried making home-brew rigidizers. I'd read some folk, who claimed to know what they were talking about, had been successful using "fumed silica", intended for thickening epoxy resins, to make rigidizer. I'd measured the density of commercial rigidizer at 1109 grams/litre. The best I could get with the epoxy thickener was 1015 grams/litre: less than one-sixth of the concentration of the commercial stuff. It seemed to work about one-sixth as effectively. I tried Sodium Silicate too (aka Waterglass). This worked very well up to about 1100 degC, 2000 degF, at which point it melted and changed from a fibre rigidizer to a fibre lubricant. Wher the burner flame hit the forge wall, the blanket just flowed away.
ITC 100 is intended as a final surface treatment (think paint). It seems to be a proprietary formula of Zirconia or Zirconium Silicate with a binder. It is not intended as a rigidizer or a deep surface layer.
If your current "rigidizer" does not give a hard surface layer, a coat or two of Satanite would not go amiss. Run the forge without ITC 100 for a bit and see how it performs before applying the ITC100 if you feel it is needed. If/when you apply the ITC100, please make no other changes at the same time and try to assess the effect it has on its own. Please let us know what you find.
