|Monthly Tech-Tip |
Modified: 2019-06-04 14:03:34
This glaze was born as a demonstration of how to use chemistry to convert a glossy cone 6 glaze into a matte.
|Ferro Frit 3124||36.00|
This is a calcium matte (as opposed to magnesia matte). As such, it develops its visual effect by the crystallization of calcium silicates (which depends on the melt being quite fluid, or runny). A fortunate side effect of the calcium matte mechanism is that additions of colorants and opacifiers can produce very interesting visuals that vary with thickness and firing cooling rate (which gets potters excited but which industry avoids because of the difficulty in maintaining consistency). However cutlery marking can be an issue with this type of glaze, especially for slower cooling kilns (where more crystallization occurs).
The spirit of this recipe originally was to demonstrate the main difference in the chemistry between a matte and glossy glaze, the SiO2:Al2O3 ratio (and how it can be changed to adjust the degree of matteness of a glaze). It was the product of a training course adjusting the chemistry of a glossy as a use-case for early versions of Digitalfire Insight chemistry software during the early 1980s). That recipe jumped from computer screens to actual use and has been employed around the world since by artist potters and even industry (especially tile).
Higher levels of MgO (as opposed to the CaO in this one) produce the other main type of true matte glaze (also well-melted yet matte), the surface of these is "wavy" that imparts a silky feel (as opposed to this calcium matte which has a smooth-feeling surface). Consider testing the G2934 MgO-matte recipe beside this one to determine which is best in your application. As with all mattes, be sure to control the rate-of-cooling in production firings (and match it to that done during testing) to get the intended degree of matteness. Remember that you can adjust the chemistry to produce more or less mattness (by lowering and raising the SiO2:Al2O3 ratio). Or you can blend in a gloss glaze base.
Because this was shrinking too much on drying (causing cracking), this recipe was changed in Mar 2019 from 37 EPK to 20 EPK and 14 calcined kaolin (although 20+14 does not equal the original 37, it is correct because the calcined version loses less weight on firing). You can make your own calcined kaolin by roasting the powder in a container in a bisque firing.
True functional mattes have fluid melts, like glossy glazes. They need this in order to develop a hard, non-scratching durable glass. The mechanism of the matte on the right is high Al2O3 (G1214Z), it is actually melting more than the glossy glaze on the left (G1214W).
Double-layered on top half. Fired with the PLC6DS schedule.
This is a calcium matte base (as opposed to the magnesia matte G2934). The clay is Plainsman M390. 5% Zircopax was added on the left (normally 10% or more is needed to get full opacity, the partially opaque effect highlight contours well). 5% tin oxide was added to the one on the right (tin is a more effective, albeit expensive opacifier in oxidation). The PLC6DS firing schedule was used.
The glaze is G1214Z cone 6 base calcium matte. 5% titanium dioxide has been added. This Plainsman M390 tile was fired at cone 6 using the PLC6DS firing schedule. Titanium can create reactive glazes, like rutile, even with matte surfaces (provided the glaze has good melt fluidity). Calcium mattes host crystallization and work particularly well. Because titanium dioxide does not contain iron oxide lighter colors and better blues are possible than with rutile. Like rutile, the effects are dependent on the cooling rate of the firing, faster cools produce less reactivity.
The body is Plainsman M390. These are commonly-used base glazes. The top one is an MgO matte, next down is a calcium matte. They react very differently to these additions. Notice also the difference when titanium dioxide is applied thickly. Tin oxide fires whiter than zircon (e.g. Zircopax). Each opacifier has issues. Tin is expensive. Titanium is difficult to mix into the slurry (screening required), not as white or opaque, variations in thickness produce more difference in results and it can turn blue. Zircon is more likely to cutlery mark, twice as much is required and it amplifies the color of any iron present.
The body is Plainsman M340S. Cone 6. Left to right: G1214Z calcium matte base glaze with 6% titanium dioxide added. GR6-A Ravenscrag base with 10% zircopax (zircon). G2926B glossy transparent base with 10% zircon (this one produces the white "Kohler Toilet Bowl" appearance we are seeking to better). G2934Y silky magnesia matte base with 10% zircon.
The body is Plainsman M390. Firing is the cone 6 PLC6DS schedule. Each horizontal row is a commonly-used base glaze. The top one is an MgO matte, the next one down is a calcium matte, row 3 is G2926B glossy and row 4 is Ravenscrag Slip+frit. The two mattes behave very differently from each other with the additions of opacifier. Thickly applying an opacified glaze will obviously affect visual character (column 4). Tin oxide fires whiter than zircon (e.g. Zircopax). If you like the G2934 recipe, consider the G2934Y variant for better melting.
These are two cone 6 matte glazes (shown side by side in an account at Insight-live). G1214Z is high calcium and a high silica:alumina ratio. It crystallizes during cooling to make the matte effect and the degree of matteness is adjustable by trimming the silica content (but notice how much it runs). The G2928C has high MgO and it produces the classic silky matte by micro-wrinkling the surface, its matteness is adjustable by trimming the calcined kaolin. CaO is a standard oxide that is in almost all glazes, 0.4 is not high for it. But you would never normally see more than 0.3 of MgO in a cone 6 glaze (if you do it will likely be unstable). The G2928C also has 5% tin, if that was not there it would be darker than the other one because Ravenscrag Slip has a little iron. This was made by recalculating the Moore's Matte recipe to use as much Ravenscrag Slip as possible yet keep the overall chemistry the same. This glaze actually has texture like a dolomite matte at cone 10R, it is great. And it has wonderful application properties. And it does not craze, on Plainsman M370 (it even survived a 300F-to-ice water IWCT test). This looks like it could be a great liner glaze.
G1214Z blue glaze from Lilly Ann Hume
G1214Z golden glaze sample from Lilly Ann Hume
The reason for this rather strange-looking wrinkling, experienced by one potter, is not obvious. But we are suspicious that the issue occurs at the application stage, and that substituting the calcined kaolin for raw kaolin in the recipe will thus resolve it. The G1214Z matte base glaze contains a high percentage of clay, kaolin to be specific. That can increase drying shrinkage enough for cracking to occur, leading to crawling. To reduce shrinkage we supply the kaolin as a mix of raw and calcined. The original recipe called for 36 kaolin, we supplied that with 20 EPK and 14 calcined kaolin. But that only adds up to 34, not 36. Why? Kaolin experiences a 12% LOI (weight-loss on firing). That means that 14 calcined kaolin supplies the same oxides as 16 raw EPK (14/0.88=16). Would this glaze work with 36 kaolin? For most people it would. As long as the bisque is porous enough to absorb water quickly, the glaze slurry does not have too much water and glaze is not applied too thickly. Adding a small amount of CMC gum, e.g. 0.1%, would also help if cracking occurs.
G1214M Cone 5-7 20x5 Glossy Base Glaze
This is a base transparent glaze recipe developed for cone 6. It is known as the 20x5 or 20 by 5 recipe. It is a simple 5 material at 20% each mix and it makes a good home base from which to rationalize adjustments.
Reducing the Firing Temperature of a Glaze From Cone 10 to 6
Moving a cone 10 high temperature glaze down to cone 5-6 can require major surgery on the recipe or the transplantation of the color and surface mechanisms into a similar cone 6 base glaze.
The Right Chemistry for a Cone 6 MgO Matte
G1214W Cone 6 Transparent Base Glaze
The process we used to improve the 20x5 base cone 6 glaze recipe
G1214Z Cone 6 Matte Base Glaze
This glaze was developed using the 1214W glossy as a starting point. This article overviews the types of matte glazes and rationalizes the method used to make this one.
G2928C - Ravenscrag Silky Matte for Cone 6
Plainsman Cone 6 Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.
G2934 - Matte Glaze Base for Cone 6
A base MgO matte glaze recipe fires to a hard utilitarian surface and has very good working properties. Blend in the glossy if it is too matte.
G1216M - Cone 6 Ultraclear Glaze for Porcelains
Substitute for low expansion cone 6 G1215U, this sources MgO from talc instead of a frit
G1214M - Original Cone 6 Base Glossy Glaze
A recipe developed by Tony Hansen in the 1980s. Its was popular because of the simplicity of the recipe and how well it worked with chrome-tin stains.
Plainsman Cone 6 Electric Standard
Used in the Plainsman lab to fire clay test bars in our small kilns
Medium Temperature Glaze Recipes
Normally fired at cone 5-7 in electric kilns.
Matte Glaze Recipes
Much less common that glossy glazes, normally have stricter firing requirements.
INSIGHT Glaze Recipes
These are sample recipes included with INSIGHT software.
Desktop Insight 2 - Creating a Matte Glaze
Learn to convert a glossy glaze into a matte by comparing its chemistry with a target matte formula. Alter the chemistry in such a way that the thermal expansion does not rise and it maintains good physical application and suspension properties.
Random material mixes that melt well overwhelmingly want to be glossy, creating a matte glaze that is also functional is not an easy task.
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