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.

Warren Frederick - November, 2000
[See manual e-mail link at the bottom of this web-page]

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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.

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BASIC FORMULA

[Parts by volume, not weight; e.g., 5 gallon bucket is 1 part]

1 part + 25% Calcium Aluminate Cement -

[Do NOT use Portland; doesn't hold up to heat]

Refcon has been used with good results; was a product of Lehigh which is now Heidelberger Calcium Aluminates, 7660 Imperial Way, Allentown, PA, 18195-1040; Phone (1-800-348-7070). One part of Refcon is the bare minimum; an actual chemical analysis varies depending on the grog size and the type/weight of other ingredients so another 25% would be useful.

2 parts Kyanite -

Calcined kyanite is mullite; 65% alumina; expands when fired; needle-like structure adds strength

Mined in Dillwyn, Virginia by the Kyanite Mining Corporation, PO Box 486, Dillwyn, Virginia 23936, Phone (804) 983-2043; only mine in the USA. Can use mullite or commercial variations such as Mulcoa (such as M47-100)-- trademark of C-E Minerals; various grades of 47% to 70% alumina; [C-E Minerals, 901 East 8th Avenue, King of Prussia, PA 19406, Phone (610) 337-7163]

2 parts Fireclay -

Have used AP Green Missouri and Hawthorn

2 parts Grog -

A variety of sizes, but large grog (e.g., 4 x 10 mesh) seems better.

+ 3 parts Sawdust -

For an insulating castable; burns out. Add damp, well-wetted after dry mixing all other (7 parts of) ingredients.

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

AP Green is now part of ANH Refractories America; Acrobat PDF files on mixing and using castables from their product page. PDF listing of installers and perhaps suppliers.

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

Refractory castables from Able Refractory Products in Houston, Texas. A discussion about castable installation and curing. Their curing/firing instructions are:

Curing The set times on Able castables, as with all castables, vary from one product to another, but generally the time required for Able dense castables is shorter than the time required for Able insulating castables. Generally, forms can be removed from side walls after twelve (12) hours for dense castables and twenty-four (24) hours for insulating castables. Forms for suspended roofs or arches require double the time of side walls.

Firing After curing is complete, a minimum of twenty four (24) hours after installation, castable firing can be started. The curing and firing process, once started, should be followed through uninterrupted.

Some moisture will still be present in the cast-able, so gradually bring the temperature to 225 F and hold for twenty-four (24) hours. This dries out the physically retained water. Note, rapid initial heating above the boiling point generates steam pressure within the castable, which destroys the strong bond and causes spalling and cracking. If excessive steaming occurs during this firing process, hold the temperature until steaming subsides. Then raise the temperature 25 F per hour to 600 F and hold for twelve (12) hours. Then raise the temperature 50 F per hour to 1200 F and hold for twelve (12) hours. Then raise the temperature 50 F per hour to the desired operating temperature. Hold at this temperature until the heat balance through the material is established to obtain the ultimate ceramic bond. Heat-up is to be continuous and uninterrupted. All temperatures are to be measured at the surface face of the refractory. Cooling should not exceed a rate of 100 F per hour.

Consult your Able Refractory sales engineer for proper procedures on more rigid cure out and firing schedules for unusually thick linings, and for any questions.