DATE: 12/10/92
CODE: 1213G

DESCRIPTION
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Base glaze adjustable cone 3-8, ultra-clear glossy and stony matte for use with stains

LOCATION
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More information available from Tony Hansen, IMC, 134 Upland Drive, Medicine Hat, Alberta T1A 3N7 FAX 403-527-7441. Fired samples of all variations location ref# 352, 461, 648

NOTES
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The above is a sample recipe of one glaze from a series done as a follow-up to previous work (series #1064). The initial objective was to produce a glossy transparent for cone 5 which would not have a cloudiness at thick areas. However, as development
proceeded, I undertook adjustments to make it work with all fifty stains I have. Then I did work to adjust its melting temperature, to reduce expansion to prevent crazing on porcelain, and finally to adjust it to produce a matte. Although each variation
might simply appear as a different glaze recipe, the common formula roots, and unified development make them all one family.

Many comments below assume you have the ability to work at the oxide level with INSIGHT and that you have read the book "The Magic of Fire".

MATERIALS
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While the formula is key, probably any materials local to your area would work as well. However, in the interests of reliability, I recommend the use of a Frit instead of Gerstley Borate to supply B2O3. The formula of each constituent material can be
deduced from the above detailed report. Be sure to do calculations to compensate for any chemical differences between your materials and those used here.

 Ferro Frit 3134 - This is a common frit available in my area, however two others, #3195 and #3124 also work well, even with a simple recipe level swap. However, the validity of any comments herein assume an oxide level substitution.

 Wollastonite - This was used as a source of CaO because it loses no weight during melting, and thus is not subject to variation in LOI as are whiting and dolomite. This also supplies SiO2, minimizing the need for flint and it is an all-around better
 material.

 Kaolin - I used a moderate plasticity kaolin as the key source of Al2O3 and adjusted things to be sure adequate kaolin is present to maintain good slurry suspension. I avoided ball clay and bentonite to source Al2O3 because they would add unwanted iron
 and possible speck.

 Flint - 300 mesh material used.

 Custer Feldspar - This feldspar was readily available to me. When selecting a feldspar, stay away from the high-soda materials.

FIRING TEMPERATURE
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The recipe as shown above melts to a clear gloss from cone 5-7. To move it downward to cone 2, move Al2O3 to .25 and the B2O3 to .3. The SiO2 can be moved up slightly to maintain glaze fit for these lower temperatures. However, the lower Al2O3 can result
in the development of some cloudiness. For cone 8, use .35 Al2O3 and 3.5 SiO2.

EXPANSION
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The recipe as shown above will fit most stoneware bodies, however the more porcelainous and the lower the temperature, the more likely crazing will be. For lower temperatures, use extra B2O3 and SiO2 to lower expansion, at higher temperatures plenty of
extra SiO2 can be tolerated. For alumina matte versions, the high Al2O3 will lower expansion, and if crazing occurs, extra SiO2 and Al2O3 in ratio can be added, or the calcium matte can be used. A somewhat higher SiO2:Al2O3 ratio can be tolerated if
necessary. As of Dec/92, I have not measured the co-efficient of thermal expansion on a dilatometer, but this will be done early in 1993.

STAINS:
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The initial impetus for the development of this glaze was to get a one whose chemistry was compatible with as many types of stains as possible, especially with the rather temperamental chrome-tin pinks and maroons. In its above form, this base works with
50 colors I have tried, and with most produces vibrant results. I did my testing and actual stain work by painting a pure stain-water mix onto the body, and applying the transparent glaze over this. One note about chrome-tin stains: keep a minimum 3:1
ratio of CaO:B2O3, 15% CaO, and don't use any MgO.

BALANCED/SAFE:
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This recipe is balanced according to normal limit formulas, although the CaO is quite high. As is, I suspect no danger of leaching, and as of Dec/92 I have not done actual tests to determine leaching of any of the metallic colorants.

VOLATILE:
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I have tested this system with varying amounts of every oxide and throughout a range of temperatures. The only aspects of volatility noted are the tendency of the ultra-clear to form cloudy areas of opacity. However, for colors and whites, this will not
be a concern.

STRONG/DURABLE:
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As of Dec/92, I have not done strength tests, but I am planning to do tensile strength tests of various clay-glaze combinations in the new year. At that time I will update this report with my findings.

BUBBLES/CRYSTALS/GLOSS/CLARITY:
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The above recipe, as shown, is a perfect ultra-clear glossy fires at cone 5-6.

Wollastonite has been used to source CaO instead of whiting or dolomite to eliminate the bubbles that appeared in earlier versions. These were associated with the release of carbonates that occurs when these materials decompose.

I interpreted the cloudy areas of opacity that nagged thicker sections on early versions of the glaze as crystallization during freezing. It appears that the key to keeping this to a minimum is keeping the Al2O3 above about .27. Leaving the Al2O3 at .25
and increasing SiO2 made the problem even worse. Moving Al2O3 all the way to .5 will produce a very nice crystal matte.

To produce a matte, I tried adding ultra-fine calcined alumina to move Al2O3 to .5, but the alumina did not dissolve in the melt. It appears it would need considerable milling to do so. To produce a silky gloss, I found .2 B2O3, .4 Al2O3 and 2.4 SiO2
worked well. A beautiful stony matte resulted from .2 B2O3, .5 Al2O3, and 2.4 SiO2. The following recipe shows an example:

  1213R ALUMINA MATTE
  WOLLASTONITE.... 24.50
  FERRO FRIT 3134. 21.00
  KAOLIN.......... 39.00
  CUSTER FELDSPAR. 13.00
  FLINT...........  3.50

An alternative way to producing a matte was a calcium saturation. This worked with .9 CaO, .17 B2O3, .43 Al2O3 and 2.0 SiO2. A sample of this variation is as follows:

  1213P CALCIUM MATTE
  WOLLASTONITE.... 33.88
  FERRO FRIT 3134. 21.10
  KAOLIN.......... 45.01

SPECIAL NOTES:
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This recipe system is still in a state of flux, and in the next few months I will have much more information on it.

PHYSICAL PROPERTIES:
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The system contains plenty of kaolin, so you could employ ball clay to source Al2O3. This would increase glaze plasticity and shrinkage for use as a slip on leather hard ware. To reduce glaze shrinkage, use a mix of calcined and raw kaolin to complete
the kaolin complement.

COST:
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I have used two moderately expensive materials, frit and wollastonite. However, this was done for good reason. I have minimized the amount of frit. In some cases, you might like to substitute in gerstley borate to supply B2O3, but make sure it is 200
mesh (pass some through a 200 mesh screen to check).

OPACIFICATION:
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I have tried a number of opacifiers, and there is no problem in producing a "toilet bowl white". Such a glaze is desirable for overglaze stain work, and the stain colors feather very nicely at their edges.

10% tin oxide: Works well but very sensitive to minute amounts of chrome in body or nearby ware which turn introduce a pinkish hue.
20% Zircopax: Excellent intense white
10% Titanium: Opaque but pinholed and yellowish
15% Superpax: Perfect white
5% Zinc: Melts more, small dimples in surface and no opacity
5% Titanium: Yellowish with just a few crystals, but not opaque.

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