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This page is the idea of Michael McDowell at mmpots@memes.com
The purpose of this page is to allow Insight users to share their hard won information regarding the expansion of their commercially produced clay bodies. As we all know, it is not possible to directly calculate expansion figures for a clay body. The various oxides of which the fired clay is composed may be combined in a variety of crystalline structures which we cannot predict. However, it is possible for us to infer the expansion of any given clay body from expansion data on the glazes that fit it under different firing conditions.
Many of us work with commercially produced clay bodies. While we can often successfully adjust our glazes to fit these bodies, sometimes this proves to be more difficult than others. In such cases, we may begin wondering what other commercially produced clays are available that have expansions more compatible with these "problem glazes". I know that this is a concern of mine, and I anticipate that I am not alone in this. It seems to me that if each of us who has found glazes that fit our current clay and firing regime could contribute information on this inferred expansion, we would then have a helpful guide for us all when we set about to search for clays with greater or lesser expansion than our current ones.
Below is a table in which I hope to present this information. To start things off, I have included my data on the two commercially produced clay bodies that I use most often. If you would like to add to it email me at mmpots@memes.com.
Experimenter |
Test Date |
Clay Body |
Manufacturer |
Fired to |
Hansen Exp. |
Roy Exp. |
M. McDowell |
Jan-97 |
Greystone |
Muddox - Sacramento, CA |
Cone 11R |
6.33 |
400 |
M. McDowell |
Jan-97 |
NBC 10 |
Clay Art Center - Tacoma, WA |
Cone 11R |
6.22 |
375 |
| T. Hansen | Jan-97 | H550 | Plainsman Clays - Alberta, Canada | Cone 10R | 6.6-7.0 | |
| Paul Wilmoth | Oct 98 | #105 White w/grog | Standard Ceramcis, Pittsburg | 04 | 6.9 | |
| Paul Wilmoth | Oct 98 | #105 Whte w/grog | Standard Ceramics | 04 | 6.76 | |
| Steve Grimmer | Oct 98 | Domestic Porcelain | Flint Hills Clayworks, Marion KS | 10-11 | 5.96 | 363 |
| Anne Worner | Nov/98 | Balcones Dark | Armadillo Clay, Austin, TX | 9 ½ - 10 | 7.04 | |
| " | " | " | " | " | 6.66 | |
| " | " | Dillo White | " | " | 6.25 | |
| " | " | " | " | " | 6.66 | |
| " | " | " | " | " | 6.76 | |
| " | " | Grolegg | " | " | 6.66 | |
| " | " | " | " | " | 6.25 | |
| " | " | Porcelain | " | " | 6.25 | |
| " | " | " | " | " | 6.66 | |
| " | " | " | " | " | 6.78 | |
| " | " | SS-6 | BlackJack Clay, Inc, Murchison, TX | " | 6.51 | |
| " | " | " | " | " | 6.66 | |
| " | " | White Stoneware | Trinity Ceramic Supply, Dallas,TX | " | 6.59 | |
| " | " | " | " | " | 6.66 | |
| " | " | " | " | " | 6.82 | |
| " | " | B-Mix ^10 | Laguna Clay Co., City of Industry, CA | " | 6.25 | |
| " | " | " | " | " | 6.66 | |
| " | " | " | " | " | 6.98 | |
| " | " | WC-370 Amador | " | " | 6.45 | |
| Don Prey | Dec 98 | B-Mix ^10 | Laguna Clay Co. | 10-11 | 6.03 | 400 |
| " | " | " | " | " | 6.65 | 450 |
| " | " | " | " | " | 7.13 | 500 |
The expansion figures in the last two columns are the calculated expansions given by Insight when the recipe for a fitting glaze is entered. Why two set of numbers? Insight calculates the expansion of a glaze based on coefficients of expansion of the various oxides that are entered in the MDT file (Materials Definition Table). The column "Hansen Exp.." is the expansion calculated for a glaze fitting the specified body fired to the specified temperature and entered into Insight using the MDT file supplied along with Insight. Ron Roy, who does quite a bit of work with Insight, uses a different set of expansion coefficients in his own MDT file that he offers for sale to interested parties. The column "Roy Exp.." is the calculated expansion when Ron's MDT is used. No one needs to have both calculated expansions in order to submit information for this table. I've included both so that people using either MDT can contribute. Obviously, the numbers are quite different, and you may be wondering which one is "correct", and perhaps what physical measurements the numbers represent. I suggest that you put both questions out of your mind. The prediction of glaze expansion is only approximate, and the numbers should not be regarded as necessarily having more than "ordinal" significance. In other words, with either Ron's or Tony's figures, it is only significant that a larger number means greater expansion.
Please consider contributing to this listing. If you have a copy of Insight and have a glaze that fits a commercially produced clay body, then enter the recipe for the glaze into Insight and record the expansion figure that Insight calculates for it. We can infer that your clay body must have approximately the same expansion.
It would be nice that before you submit information for this project that you put the fit of your glaze to a test. The test I recommend is to make a small cup out of the clay body and give just the inside of the cup a medium thick application of the glaze you believe fits. After firing in your normal way, leave the cup in your freezer over night. In the morning, boil some water. Then take the cup out of the freezer and immediately fill it with the boiling water (It might be a good idea to set the cup in the bottom of your sink to do this). If you have a good fit between clay and glaze, there should be no visible crazing resulting from this stress.
If you have a glaze that passes such a test, then I don't need to know the formula of the glaze, just the calculated expansion that is given by Insight, the name and manufacturer of the clay, and the temperature and atmosphere at which you fire. This page will only really become useful as the number of entries in the table grows. It's up to you now...
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