Over the last fifteen years this refractory castable formula has been used to create the arch tops of five anagama kilns in Virginia and Maryland. Three arch tops are of similar size, approximately four to five feet interior height, five feet wide, and 18 feet in length. The fourth and largest arch top is about triple in volume; it spans about seven feet in width, up to seven feet in height, and is 35 feet in length. The fifth is a hybrid: a 12-foot long barrel arch in front of two noborigama chambers.
These notes are intended to share our knowledge, but obviously use of this information is not our responsibility--the actual ingredients used, the methods and quality of mixing, and the amount of water added are all critical components.
For one commercial source--which was used for the largest kiln in order to ensure completing the arch in one day--of premixed refractory castable, contact Craig Felton, Director of Technical Services, Mt. Savage Specialty Refractories, PO Box 608, Mt. Savage, Maryland, 21545, Phone (301) 264-3595.
Frederick - November, 2000
Special thanks to Bill Knoble of Red Truck Clay Works in Chestertown, New York--one of the original wood-firers--for his initial efforts and experience in researching and developing this castable information twenty years ago.
[Parts by volume, not weight; e.g., 5 gallon bucket is 1 part]
1 part + 25% Calcium Aluminate Cement -
2 parts Kyanite -
2 parts Fireclay -
2 parts Grog -
+ 3 parts Sawdust -
MIXING AND APPLICATION
Dry mix 1.25 part cement + 6 parts other. Have mixed by hand/hoe in wooden trough. Can also use clay mixer for dry mixing only; we have split apart a pug mill by adding water--the castable sets up too fast and the pug mill cannot be cleaned between batches.
Get sawdust ready; try to maintain an even dampness. Add sawdust to dry mixture. Add water to obtain a firm, "ball-in-hand," plastic consistency.
One team applies the castable over a form (which is covered with plastic for good curing) while other teams continue to mix up batches of castable. We have had teams of 10 to 40 people. It is useful to complete the arch in one work session so it cements together as a monolithic unit.
Use castable like structural brick; work with it like clay. Work perpendicular to the form; create slight wedge shapes so that whatever cracking/layer separation occurs is structurally immaterial. In essence, you are really casting bricks by hand and in place. Setup time depends on the weather and the mix; minimum available time between mixing and applying is usually 15-25 minutes.
Normally we make at least a five inch thick layer of good castable. Over this another layer of "junk" castable is added composed of clay, sand, and cement. Have used: 3 parts silica sand; 1 part clay; 1 part cement; and 3 parts sawdust.
Assume 100 lbs of material are required for 1 cubic foot of castable. [So a kiln with 100 square feet of surface area would require 50 cubic feet of material to make a 6 inch thick layer, or 5000 pounds of material.] Don't figure too closely; for instance, assume materials are required for a 6" thick layer even though you might be aiming for 4 1/2"
The kiln illustrated here required approximately 1000 lbs of fireclay; 1300 lbs. of kyanite; 1000 lbs. of grog; 500 lbs. of Refcon, and a pickup truck of sawdust.. Note that Kyanite is particularly heavy, so some 30% more was required when calculated by weight.
TECHNICAL INFORMATION LINKS
An overall, detailed explanation of high alumina cements is in the September 1998 issue of Betoniek from Heidelberger Calcium Aluminates; choose <Bibliography> then <Betoniek> to download a PDF file (readable with the Adobe Acrobat reader) or a ZIP file.
Calcium aluminate cements achieve 80% of their strength within 24 hours; Portland cement requires 28 days to develop the equivalent percentage strength.
Technical information on concrete from QCL in Australia; can't link directly because of frames, so choose <Technology> then <Technical Notes>
History and principles of concrete from MAST, Materials Science and Technology Teacher's Workshop, Prepared by the Materials Science and Engineering Department at the University of Illinois, Urbana/Champaign
Lafarge Aluminates for calcium aluminate cements
Whittaker, Clark & Daniels for calcium aluminate cements (older link with summary table) newer link to individual info sheets on three grades
questions to: Warren Frederick
<wf> at <artistpotters.com>
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