|Monthly Tech-Tip |
Orange peel is a defect or physical property of ceramic glazes
Ceramic glazes can have a surface texture like orange peel, this can be intentional or a defect. Matte glazes, depending on their mechanism, can have a smooth but dull or a bumpy but glossy surface (the bumps are very fine and scatter the light to produce a silky matteness). Variegated glazes having phase differences can also have an orange-peel surface. Glazes with significant entrained bubbles and those not sufficiently melted can also have a rougher peel surface.
Assuming the glaze is sufficiently melted, a firing cycle having a slower cool can help level the surface of glossy glazes. If better melting is needed the recipe can be adjusted to have more fluxing oxides. Adding some frit can really help (e.g. Frit 3195 has balanced chemistry and will often help melt glazes with minimal impact on other properties). If the glaze also has issues with crazing Ferro Frit 3249 can help solve both problems. If the rough surface is caused by excessive gas bubbles being generated during firing then adjusting the glaze recipe to source the chemistry from materials having a lower LOI can really help (e.g. Gerstley Borate and frits both source B2O3 but the former has a very high LOI). Carbonate materials can be a big source of gas from right within the glaze, generating millions of orange peel producing bubbles. If the body is gassing then a drop-and-hold at the end of the firing will really help. Slow cool from there will help even more. If the glaze has a stiff melt (e.g. is high in Al2O3) the chemistry can be adjusted to raise the SiO2 and drop the Al2O3. High MgO glazes naturally have an orange peel surface (MgO is from talc and dolomite). That being said, some MgO can be tolerated, in ultra gloss surfaces, but when a threshold is reaching the surface texture is affected.
Sometimes the best approach is to transplant the fired mechanism-producing materials in the recipe (the ones giving the color, variegation and opacity) into another base transparent that works well on your clay bodies.
This transparent glaze adds a little manganese and iron, just enough to give color, but still maintain transparency to highlight the decorative crack-network in the engobe below. However this glaze is not as brilliant and transparent as it could be. As apparent in the surface reflections, it has somewhat of an "orange peel" texture on the glass surface. This is due to a combination of factors (e.g. not enough melt fluidity, gassing of the manganese during melting, cooling the kiln too quickly). If the colorants were transplanted into a more fluid-melt transparent, this glaze could be improved. Photo courtesy of J. Decker.
This is a cone 10 glossy glaze. It has the chemistry that suggests it should be crystal clear and smooth. But there are multiple issues with the materials supplying that chemistry: Strontium carbonate, talc and calcium carbonate. They produce gases as they decompose, if that gas needs to come out at the wrong time it turns the glaze into a Swiss cheeze of micro-bubbles. A study to isolate which of these three materials is the problem might make it possible to adjust the firing to accommodate it. But probably not. The most obvious solution is to just use non-gassing sources of MgO, SrO and CaO (which will require some calculation). There is a good reason to do this: The glaze contains some boron frit, that is likely kick-starting melting much earlier than a standard raw-material-only cone 10 glazes. That fluid melt may not only be trapping gases from the body but creating a perfect environment to trap all the bubbles coming out of those carbonates and talc. The Aero chocolate bar of glazes!
The cone 03 porcelain cup on the left has 10% Cerdec encapsulated stain 239416 in the G2931K clear base. The surface is orange-peeled because the glass is full of micro-bubbles that developed during the firing. Notice that the insides of the cups are crystal-clear, no bubbles. So here they are a direct product of the presence of the stain. The glaze on the right has even more stain, 15%. But it also has a 3% addition of Zircopax (zircon). Suppliers of encapsulated stains recommend a zircon addition, but are often unclear about why. Here is the reason: It is a "fining agent".
A kiln firing schedule where temperature is eased to the top, then dropped quickly and held at a temperature 100-200F lower.
Plainsman Cone 6 Slow Cool
350F/hr to 2100F, 108/hr to 2200, hold 10 minutes, fastdrop to 2100, hold 30 minutes, 150/hr to 1400
|By Tony Hansen|
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