|Monthly Tech-Tip |
Modified: 2020-12-02 07:29:04
A cone 03-02 low expansion clear ceramic glaze developed from Worthington Clear.
|Ferro Frit 3195||19.00||KG||19.1|
|Ferro Frit 3249||19.00||KG||19.1|
|Fusion Frit F-524||49.00||KG||49.2|
This is a low-expansion version of the Zero3 Clear glaze (code G2931K), originally developed from the Worthington Clear Gerstley Borate based recipe. While G2931K fits the vitreous Zero3 porcelain and non-vitreous bodies containing talc (like Plainsman L212, L215), it crazes on others (e.g. Plainsman TerraStone, BuffStone, L210). This version drops its expansion drastically, from 7.4 down to 6.3 (by adding ZnO, ZrO and increasing MgO and SiO2).
Since the thermal expansion of this is higher than the original Wothington Clear and the L version is lower, it is thus possible adjust thermal expansion of either recipe by adding some of the other (to deal with shivering or crazing on almost any low fire body).
Do not try to use this with too little water. Mix it to 1.43 specific gravity and add vinegar or Epsom salts to gel it so that it stops moving (after vigorous stirring) in 2 or 3 seconds. When mixed right it applies very well to low or high porosity bisque (in a very thin layer if needed), it does not drip or move.
The hold:slow-drop firing schedule, like 04DSDH, is critical to a defect-free surface.
Glaze fit. The left-most clay mug contains no talc (Plainsman Buffstone), the centre one about 25% talc (L212) and the right one is about 45% talc (L213). Talc raises thermal expansion. The centre glaze is G2931K, it is middle-of-the-road thermal expansion (Insight-live reports it as 7.4) and fits the low-talc bodies (and Zero3 porcelain and stoneware). But it crazes on Buffstone and shivers on L213. The lesson is: Forget about expecting one clear or base glaze to fit all low fire bodies. But there is a solution. I adjusted it to reduce its expansion to work on zero-talc porous bodies and raise it to work on high talc bodies. How? By decreasing and increasing the KNaO (in relation to other fluxes). The three fire crystal clear and work the best in a drop-and-hold firing.
Replacing the Gerstley Borate in recipes containing 50% or more of it
Plainsman Zero3 Glaze L,K,H and Firing Schedule
Zero3 porcelain/stoneware, engobe, clear glaze and firing schedule
G2931H - Ulexite High Expansion Zero3 Clear Glaze
A cone 03-02 higher expansion fritted clear glaze developed from Worthington Clear.
G2931K - Low Fire Fritted Zero3 Transparent Glaze
A cone 03-02 clear medium-expansio glaze developed from Worthington Clear.
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|By Tony Hansen|
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