Saturday, September 4, 2010

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 , or reply in the comments.

It takes a village to raise a kiln. 

1 comment:

Tony said...

Maybe Alex Matisse could help you :