Orange Peel Surface
Orange peel is a defect or physical property of ceramic glazes
Details
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 rippled 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 (or equivalent) 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 generate lots of orange peel, producing bubbles at the wrong time. Strangely enough, stains can be a source of bubbles also, try a different one to be sure. 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 may be able to 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 reached, 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 will can also resolve another possible issue: A bad shipment of frit. A different clear base that uses a different frit would be a good test to verify that.
Related Information
A honey glaze that needs a base having more melt fluidity
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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.
Orange-peel or pebbly glaze surface:
How to make it on purpose
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Orange-peel surfaces are normally regarded as a glaze fault, but they can also be created intentionally. The term describes the finely dimpled or pebbled surface rather than one specific cause.
One way to produce it is to trap bubbles in a glaze melt. Gas can come from the glaze materials, the body or an underlying engobe or underglaze. In the glaze itself, raw carbonates such as strontium carbonate, barium carbonate, magnesium carbonate, dolomite and calcium carbonate can generate substantial gas. Substituting these for fritted or otherwise low-LOI sources while maintaining approximately the same fired chemistry can increase bubbling.
The melt must be viscous enough, or remain fluid for too short a time, for the bubbles and dimples to survive. This can be encouraged by increasing Al2O3 or SiO2, firing only to the beginning of glaze maturity, eliminating the peak soak and cooling quickly through the range where the glaze can still level. Surface tension also influences wetting and bubble behaviour, but high surface tension alone does not necessarily trap bubbles.
A second method is to preserve texture already present in the unfired coating. Spraying can deposit a stippled layer; brushing, sponging or applying glaze over a textured body can do the same. A sufficiently stiff or late-melting glaze will retain this relief instead of levelling completely. Since no internal bubbles or second phases are required, this method has the best chance of retaining transparency.
Orange peel can also develop during cooling as crystals or phase-separated structures roughen the surface. Salt and soda vapor firing produce another classic form: sodium vapor reacts with the silica and alumina of the body or slip to create a thin, naturally dimpled glaze. Layering glazes having different viscosities and wetting behaviours can also create similar surfaces, although the results are often very sensitive to thickness and firing conditions.
Zircopax as a fining agent to de-bubble a stained glaze
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The cone 03 porcelain cup on the left has 10% Cerdec encapsulated stain 239416 in the G2931K clear base (this is not an underglaze with a transparent over it). The surface is badly 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%. Why isn't it even worse? It also has a 3% addition of Zircopax (zircon), which acts as a fining agent. As you can see, this makes a dramatic difference. Suppliers of encapsulated stains even recommend a zircon addition, but are often unclear about why. This is the reason.
The yellow on the right could be improved further with a slow-cool or drop-and-hold firing. And if the glaze was reformulated to use a later melting frit (so fewer bubbles even get trapped).
Glaze with an encapsulated stain is bubbling. It needs Zircopax.
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These two pieces are fired at cone 6. The base transparent glaze is the same - G2926B Plainsman transparent. The amount of encapsulated red stain is the same (11% Mason 6021 Dark Red). But two things are different. Number 1: 2% Zircopax (zircon) has been added to the upper glaze. The stain manufacturers recommend this, saying that it makes for a brighter color. However, that is not what we see here. What we do see is the particles of unmelting zircon acting as seeds and collection points for the bubbles (the larger ones produced are escaping). Number 2: The firing schedule. The top one has been fired to approach cone 6 and 100F/hr, held for five minutes at 2200F (cone 6 as verified in our kiln by cones), dropped quickly to 2100F and held for 30 minutes.
Encapsulated stain orange-peels the glaze?
Why? What can be done?
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This is happening on five different stains at 8% concentrations. The body: A fritted porcelain. Temperature: Cone 03. The glaze: 85% frit. The solution? Documentation for inclusion frits notes that adding 2-3% zircon can brighten the color. Although this does not seem intuitive, we added 2% anyway and refired another sample. You can see the dramatic difference on that tile below. The color is brighter because the micro-bubble clouds that were diffusing it are gone! Of course, it is apparent that the percentage of stain also needs to be increased to get more intense color. What happened to the bubbles? It could be that the particles of zircon that float, unmelted in the glaze melt, act as seed-points for bubble agglomeration and the bigger bubbles then break the surface and it heals behind them. But where do the bubbles come from? I do not know.
Links
| Glossary |
Drop-and-Soak Firing
A kiln firing schedule where temperature is eased to the top, then dropped quickly and held at a temperature 100-200F lower. |
| Firing Schedules |
Plainsman Cone 6 Drop-and-hold, Slow Cool
350F/hr to 2100F, 108/hr to 2200, hold 10 minutes, fastdrop to 2100, hold 30 minutes, 150/hr to 1400 |
| Materials |
Copper Carbonate Basic
This form of copper carbonate is the article of commerce, a mixture of theoretical copper carbonate and copper hydroxide. |
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