we just had two cast core failures. The material did not harden, crumbled. Part of our confusion is with product names. What is fireclay, furnace cement, refractory cement, and refractory mortar? They go by different names and descriptions store to store! Please help!
Bob Cloud

Robert; Most common brand of "fireclay " is lincoln 60, another is hawthorn. fireclay is a powdered clay sold in 50 bags with properties known for high heat resistance. Furnace cement is generally used to seal leaking conventional wood stoves, and would not make a good additive to a core mixture. Refractory cement and refractory mortar are probably a very similar product, either should work as an additive to your fireclay / perlite mix commonly used to cast a core (walker style). Adding sodium silicate (water glass)or painting it on afterwards will also harden up your casting.No concrete products can stand up to the heat of a rmh core,only refractory products can take the high heat. My complaint with a cast core is the frailty of the feed tube area, mine went from a 7.5" square to the size of a 5 gal pail by the end of one burning season. Cast cores burn super hot extra fast but... they are fragile and will crumble if you treat them roughly. I now build my cores from heavy fire brick , they take much longer to heat up but are very durable.

Thanks for the info. What kind, brand of firebrick would you recomend, what temp range?

Bob; All my firebrick was acquired as salvage , for free or trade. Different names stamped on them, i've gotten soft insulating brick and lots of heavy brick. From research it seems most firebrick runs from 2300 F - 3000 F. I live in the Pacific NW and sawmills are everywhere. They all use boiler fed dry kilns to dry their lumber. When a sawmill closes it all comes down and an auction house sells all the parts from the mill ,including (you guessed it) pallets of firebrick. Most buyers at these auctions only want certain items but have to buy pallets of other stuff like.... firebricks, if you ask they may say "go dig them up and their yours!" The White Block store in spokane valley, washington sells lincoln 60 fire clay at $9.00 a bag and also sells brand new whole and split firebrick but I never looked at the prices.

I am just adding some more info to roberts post on the casting failure. The clay we used was fire clay bought from a pottiers supply store. I beleive it had a cone 10 rating and was said to be good for 2500 degrees. The first mix we did nt use this fire clay and had the same results. The second mix was mostly fire clay (200lb dry) one bage +/- vermeculite and two pails of refractory cement. After 24hrs the casting was fired, the second day the casting was fire again. Sever cracking occured and when the sides were removed, the casting was cracked so bad that it will not hold together. I have watched a vid on youtube describing this method of casting twice and can not figure out what went wrong. Please help!

Nowhere near enough time for monolithic casting to dry. For 200 pounds, i think you could wait for a good two months to dry. What you had is steam cracking. The water turned to steam, and wanted to escape. IIRC, one drop of water can take 400 hundred times it's original volume, when turning to steam. So guess what you get? Cracks.

Bob; To start , nobody who casts their own core can seem to wait more than a few days till firing it off ! Perlite is highly recommended over vermiculite , vermiculite holds water , leading to steam . Some cracking is to be expected, just not the excessive cracking you are getting . After all you are going to cover it with cob. 200# clay with one bag of vermiculite & 2 buckets of refractory seems like a very clay rich mixture. If I recall, I used 100# of fireclay with 4 cubic feet of perlite and one bucket of refractory. I did have feed tube issues and at the very end of the burning season a few inches of my burn tunnel roof collapsed. If I was to cast another core I would use a compleat insulated refractory mix rather than fireclay & perlite. After casting and just before firing I would paint the whole feed tube area with water glass , in the hope that it would make it more durable. Risers on the other hand do not need much (if any)refractory, only fireclay and perlite. Last photo shows a new one just about ready to be covered by my 55.

Thanks for the info! This site has been a great resource! We are going to buld a new J tube with fire brick. Should we go with dry stacking or mortar the joints? We would cover the dry stack with clay/ But if we use mortaer what kind is reccomended?
Thanks again
Bob

Bob: No to dry stacking. Use fireclay as your mortar. Some people make a slurry in a bucket and dip each brick into it as they build. I prefer a slightly thicker slurry and use my hand to smear onto each brick. As long as you keep it thin either way works fine. Try to find a named powdered fireclay like (lincoln 60) it's heat resistant properties make it expand or contract less than other clays.