clay additives

I'm trying to come up with something (cheap and available) to line the inside of the heat riser with to make it more durable. I was thinking clay/wood ash. I talked to my clay supplier yesterday. He said wood ash acts as a flux, like feldspar. He gave me some "washed ash" saying that would have different properties than plain ash. Okay? Why is it getting more complicated rather than less? He showed me some test tiles he'd made mixing boric acid into the clay. The one at ten percent looked good. At fifteen it had flowed out and made a glassy obsidian like coating, at twenty it had bubbled out and made a mess.

My wife isn't going to let me use our one good kiln to run my tests. Looks like I'll have to make a rocket stove fired kiln to run my tests in. On the bright side she found some cheap used kiln shelves at a pottery supply that that I should be able to use to make a top notch fire tube liner.

She's been showing me, in one of her books on glazes, how iron oxide does some very different things at different temperatures. I'm still not sure exactly how hot the riser is going to get... so what temp. should I test to? Doesn't anyone out there have this already figured out?

The funny thing was that he didn't have any clay ready on hand, and it was freezing rain so we couldn't go out and dig some, so I came away with additives to try but no clay to try them in, much less to make my test kiln from. I'm really ready for this winter to be over.

Tom; Is your riser already built with fireclay & perlite ? If it is not, use a piece of sonitube as a sacrificial liner and after your first firing you can paint the inside with sodium silicate (waterglass) this will seal the riser with a glass like coating. I used this on my cast core in the feed tube side, as i was getting alot of damage from the wood being inserted. It did not stop my problem but it really slowed it down. I do not think that it is necessary in the riser but that is just my opinion . Waterglass is readily available on ebay, at a reasonable price. Fireclay comes in 50# sacks and can usually be found at a building supply. If the place does not know what fireclay is then try another, it took me 3 stores, 2 of witch wanted to sell me bentonite and had never heard of fireclay. The last store had a pallet of it for 15$ a bag. Perlite not vermiculite i found at a nursery / landscaping place. Hope this helps. Tom

Yes, Thanks. Waterglass was on my list of things to try, you just moved it up the list. I want to try mixing it in along with the other additives. My first project will be a stove like this, http://www.rechoroket.com/Home_files/EnglishBook.pdf Something I can use to heat a kiln/pizza oven, or even just in canning. I don't think they make sonitubes that small, but maybe a carpet roll. I want to use slab rolling techniques to make a liner to wrap around the core as a hardface, then pack the clay/perlite mix around that. This first one will just be to practice the technique. When I can use it to fire some test tiles I'll get a better idea of what I want to do when I build my RMH.