|Monthly Tech-Tip |
Modified: 2020-12-15 14:36:48
A chrome-tin burgundy glaze using the Ravenscrag cone 6 base recipe.
|Ravenscrag Slip 1000F Roast||30.00|
|Ferro Frit 3134||20.00|
Chrome-tin (either from the raw materials or a stain) pink and red glazes can be difficult to achieve and keep consistent at cone 6. Red is perhaps the most difficult and expensive color in ceramics. The chemistry of the host glaze has to be sympathetic to the color development, the chrome and tin require high calcium, zero zinc and low boron (that is why this recipe has 10% added calcium carbonate).
Remember that red is red, it is difficult to acheive in glazes; test this first on different kinds of clay to determine if is is suitable for you. It may be better for you to use a pink or maroon stain instead instead of raw chrome and tin (e.g. the GR6-L recipe).
Try different thicknesses to see which works best for you. This breaks clear around edges to highlight irregularities in the surface. On darker bodies the color of the glaze will be darker. Consider reducing the percentage of colorant to get a lighter color if needed. Slow cooling can matte the surface.
For mixing and firing instructions please see the master recipe, GR6-A.
The GR6-L (the two on the left) employs raw chrome and tin, the GR6-E uses a stain (Mason 6006). The L does not melt quite as well (because of the 10% whiting in the recipe to make sure it develops the chrome-tin color well). The GR6-E is does not need the whiting. The E is much more flexible because one can choose different stains to get different colors. Of course the intensity of the color can be adjusted by varying the percentage of colorant. And the L could be made to melt better by increasing the percentage of frit.
These are Plainsman P300 mugs fired at cone 6. When the glaze, GR6-E, goes on too thick (as on the left) it is dark maroon and has a pebbly surface that does highlight contours. This went on too thick because the specific gravity of the slurry was too high, about 1.53 (even a one-second dip put to thick a layer on the pieces). When I thinned it down to about 1.45 and flocculated it using espom salts, it went on thinner, yet still evenly, and I got the result on the right.
The outer glaze is Ravenscrag GR6-E Raspberry, the bright maroon color is a product of the surprising interaction between the 0.5% chrome oxide and 7.5% tin oxide present. That small amount of chrome is only enough to give the raw powder a slight greenish hue, hardly different than the clear liner glaze. While this color mechanism appears to be effective, it is delicate. A maroon stain is actually a better choice. It would fire more consistent would be less hazardous to use. And the raw glaze will be the same color as the fired one!
The body is Plainsman M340 and these two glazes are based on the GR6-A recipe (Ravenscrag Slip + 20% frit). The GR6-C creamy white glaze adds 10% Zircopax to opacify it. The pink version, our code number GR6-L, adds Mason 6006 stain instead. The GR6-A base is zinc-free and just hits the 10% minimum CaO recommended to get color development with a chrome tin stain. This recipe also couples a low MgO level (MgO can kill the color in chrome tin stains).
GR6-A - Ravenscrag Cone 6 Clear Glossy Base
This Plainsman Cone 6 Ravenscrag Slip base is just the pure material with 20% added frit to make it melt to a glossy natural clear.
GR6-L - Ravenscrag Cone 6 Transparent Burgundy
A stain-based method to achieve this color using the Ravenscrag base recipe.
Medium Temperature Glaze Recipes
Normally fired at cone 5-7 in electric kilns.
Ravenscrag Slip Recipes
Recipes based on Ravenscrag Slip from Plainsman Clays.
Plainsman Cone 6 Electric Standard
Used in the Plainsman lab to fire clay test bars in our small kilns
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|By Tony Hansen|
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