And Now a Word about Castables

Warning: this entry will likely be hideously boring to non-potters. My apologies. Sometimes I note things here so I know where to find them!

Several words, actually. Turns out to be a more complicated subject than I thought, and very little information is available on the web about castables. It's worth noting that two of the three potters I consulted recommended commercial castable, specifically mizzou, if it is to be used for anything structural, like, say, casting the arch itself. I was able to score four (or four and a half, depending on how you look at it) recipes for insulating castable, such as you would use to put over a brick arch, to slow heat loss. I was quite surprised by how widely differing the recipes are.

Tim Cichocki, a potter from Norrgidgewock, Maine, used this recipe to insulate his groundhog kiln:

Equal parts:

• Fireclay
  
• Sawdust
• Silica Sand
• Portland Cement

From Tyler Gulden, Executive Director of the Watershed Center for Ceramic Arts:

• 12 parts crushed soft brick
• 7 parts fireclay (Hawthorn Bond)
• 1 part Portland Cement

Tyler also offered a more durable, less insulating version:

• 10 parts crushed soft brick
• 9 parts fireclay
• 1 part Portland Cement

From Reeder Fahnestock, Watershed's Facilities Director:

By weight

• 4 parts Fireclay
• 2 parts Alumina
• 1 part sand or grog
• 1 part crushed soft brick
• 1 part Portland Cement

I found two castable recipes on the web from Goshen College in Indiana. This page contains a very thorough treatment of the subject, and more complicated than my needs require, what with all the laminating of the hot-face and insulating mixes. For my door, I am only casting hot-face blocks, and using soft brick outside them. The recipe that I need for this purpose is:

Door Brick Castable Recipe for Soda: Non-Insulating

• 10 parts Kyanite
• 1 part Ball Clay
• 2 parts Fireclay
• 3 parts Kaolin
• P-grog (a high heat duty grog, sold by AP green)

I will be applying the insulating castable directly to the exterior of the arch, so of course there is no way to fire it; therefore Portland Cement is useful is persuading it to set. Portland Cement melts at ^10, however, so I decided to steer clear of any hot-face recipe that contains it. ( I know, I know -- lots of highfire claybodies contain material that fluxes at ^10, it all depends on the proportion, blah, blah, blah. I guess I just don't trust it. Plaster has never been my friend in the studio.) I am leaning towards the recipe which is heaviest on the crushed firebrick, for my insulating layer, mostly because I have  a whole boatload of rubble softbrick, and it seems a shame to waste it. I just have to pulverize it. 

Applying castable will also be a learning experience for me. Specifically I am wondering how long it will stay workable. Should I rent a small concrete mixer to keep it from setting? Or mix up small batches, only as much as I can apply before the set time. What is the set time, anyway?

If you have the answers to these or any of life's persistent questions, shoot me an email at castable@finemesspottery.com , or reply in the comments.

It takes a village to raise a kiln.