All Glossary

Glaze thickness

Many ceramic glaze benefits and issues are closely related to the thickness with which the glaze is applied. Many glazes are very sensitive to thickness, so control is needed.

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Simple example of color-variation-by-thickness in a honey glaze
This is GA6-B, a transparent amber glaze at cone 6. The color darkens in the recesses as a simple function of thickness.

There is no rule about how thick glazes must be applied to ware. It is about experience and testing. There is no way to avoid losing ware as one learns what is too thick and too thin for each type. At a minimum, learn to recognize when thickness is right or wrong and adjust future technique accordingly. It doesn’t matter how you get the glaze on the ware, as long as it is the right thickness and is sticking on well enough to hang on during firing.

Even application and getting the right thickness are quite tricky with brushing glazes. The method and number of coats needed depend on the condition of the slurry, this can vary by batch and age. Sometimes glazes are like water, sometimes like syrup. So it is a matter of judgement to target credit-card-thickness for most common glaze types. Dipping glazes are easier to get on evenly but trickier to get the right thickness. One must pay attention to the specific gravity and thixotropy, the type of clay and the bisque temperature. Experienced workers tend to gauge thickness simply by eyeballing the slurry and controlling the dipping time. Spray glazing, that’s a whole other thing, getting the right thickness is just pure experience.

There is no end in experimenting to learning different visual effects that can be created by thickness variations and glaze layering. Experience is the teacher in learning to recognize when the thickness being applied is right. Experience is also the teacher in learning how bisque firing temperature or wetness (and therefore its porosity), duration of dipping, the number of brush coats applied; the viscosity, thixotropy and specific gravity of the slurry; the percentage of plastic clays and gums in the recipe, etc all affect thickness.

Ware type determines thickness needs. On some types (e.g. transparently glazed fine porcelain) the glaze must be thin and very evenly applied. On others (e.g. majolica) it must be extremely thick to produce an opaque, even white. By contrast, the shade of transparent colored glazes varies with thickness - potters exploit this by incorporating surface contours and textures on the ware to purposely encourage thickness variation.

A number of glaze defects are directly related to glaze thickness.

• Glazes can pinhole or exhibit a rough texture if applied too thinly on bodies having coarser particles and/or high LOI.
• Fluid melt glazes, or those having high surface tension at the melt stage, can blister on firing if applied too thick. Or, run off ware.
• Glazes having sufficient clay to produce excessive shrinkage on drying will crack (and crawl during firing) if applied too thick.
• Glazes having a thermal expansion lower than the body, and thickly applied on the inside of vessels, can fracture the piece during kiln cooling (those having a higher expansion than the body are more likely to craze).
• Thickly applied transparent colored glazes will fire the wrong shade.
• Transparent glazes applied too thickly will often cloud, obscuring underglaze decoration.
• Heavily opacified zircon glazes have reduced melt mobility and, if applied too thickly, will crawl on abrupt concave contours.
• Low-temperature glazes have a less developed bond with the body and can shiver around sharp contours if applied too thickly.

Certain visual effects depend heavily on the thickness of glaze application. Reactive glazes that variegate or crystallize usually require a specific thickness (and firing curve). Crystalline glazes must be applied too thick so they will run during firing. Underglaze decorated porcelain requires that the transparent overglaze be applied as evenly as possible. Majolica glazes, as noted above, must be applied thick. Bodies containing coarse particles require thicker glaze coverage to produce a smooth surface.

In industry, glaze thickness is just a production parameter that must be adhered to, machines are calibrated, and glaze slurries are tuned to specific specifications. People working at a factory may thus never see a piece whose glaze thickness is not right.

Related Information

Thick application clouds this transparent glaze:

On terra-cotta bodies, this is an issue

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Glaze clouding is a universal issue in ceramics. Terra cotta bodies often demonstrate this well. Pretty much all transparent glazes, even commercially available ones, can cloud, especially when applied thickly. This example is G2931K, it can be beautifully crystal clear when thin. But, as thickness increases, this happens. We ball-milled it to see if that would help, but as you can see, that has not impacted the problem. This is a dipping version, so that is part of the reason why it is easy to get it on too thick.

One of the advantages of brushing glazes is the ability to carefully control thickness. Many are formulated with a low specific gravity, requiring multiple layers for adequate thickness. Two coats rather than three are often better for this type of body.

An Ikea mug demonstrates stoneware glaze thickness

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Potters can learn this from a commercial manufacturer. This stoneware mug was bought at Ikea. The body is a highly vitreous bone-colored stoneware, likely fired to around 2200F (1200C). The inner glaze is a dark amber transparent (similar to GA6-B), and the outer is a floating blue (similar to GA6-C or G2917). The fired glaze thickness is about 0.5 mm inside and out. They are wise to keep the thickness to a minimum on the inside to avoid glaze compression issues.

Where transparent glaze thickness really matters:

Over this Amaco velvet underglaze

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At the leather-hard stage, the sides of these two L4410P low-temperature dolomite body pieces were coated with AMACO velvet underglazes. Both were bisque fired and finished with a layer of the same transparent glaze. But the difference is the thickness of that glaze and the method of application: The one on the left got three thin layers of a brushing glaze. The one on the right was quickly dipped in a base coat dipping glaze version. Evidently, there is a thickness threshold, which, when exceeded, results in clouding. We have observed that this happens with every clear glaze we have tried.

For even coverage white majolica glazes must be applied by dipping

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The mug on the left has three coats of Spectrum Majolica brushing glaze. Drying was required after doing the inside coats, so the total glazing time was several hours. The glaze layer is way too thin, and it is quite uneven! The one on the right was immersed in a 5-gallon bucket-full of G3890 Arbuckle dipping glaze white (that was weighed out according to a recipe and slurried at 1.62 specific gravity). It took seconds to dip-apply, the thickness coverage is good. As is obvious, it makes sense to make your own base white. Then decorate using the overglaze colors (e.g. the Spectrum Majolica series). Another advantage of a DIY white is that you can splurge on the amount of opacifier (in this case 9% zircon and 4% tin oxide), to achieve maximum whiteness and opacity. And, you can proportion a mix of two frits (having higher and lower thermal expansion) to fine-tune the fit with the body (a big issue at low fire).

A breaking glaze highlights incised decoration:

The mechanism is variation in thickness

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This is the Ravenscrag slip cone 6 base (GR6-A which is 80 Ravenscrag, 20 Frit 3134) with 10% Mason 6006 stain (our code GR6-L). Notice how the color is white where it thins on contours, this is called "breaking". Thus we say that this glaze "breaks to white". The development of this color needs the right chemistry in the host glaze, and it needs depth to work (on the edges the glaze is too thin so there is no color). The breaking phenomenon has many mechanisms; this is just one. Interestingly, the GR6-A transparent base has more entrained micro-bubbles than a frit-based glaze; however, these enhance the color effect in this case.

French Émail ombrant technique:

It highlights design by glaze thickness alone

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"Émail ombrant" (French for “enamel shadow”) is a pottery-decorating technique developed in France in the 1840s (at the Rubelles factory by Baron A. du Tremblay). Designs were etched or stamped into the pottery, and a transparent colored glaze was applied thickly enough to re-level the surface. The varying depths produced colour highlighting.

This boron blue effect depends on three things:

A dark body, variations in thickness, the right chemistry

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This is G2826A3, a transparent amber glaze at cone 6 on white (Plainsman M370), black (Plainsman 3B + 6% Mason 6666 black stain) and red (Plainsman M390) stoneware bodies. When the glaze is thinly applied, it is transparent. But at a tipping-point-thickness, it generates boron-blue that transforms it into a milky white. Glazes that are very glassy but on the edge of structural instability do this. So they are not good for functional ware.

This is an adjustment to the 50:30:20 Gerstley Borate base recipe (historically used for reactive glazes, often on functional surfaces! This cuts B₂O₃ and adds significant SiO₂. But it still has double the boron of a typical functional glaze. While the chemistry of the original was within the territory of boron blue development (relatively low Al₂O₃), this one is better because of the increased SiO₂ (the high MgO:CaO ratio is likely also helping). Boron blues like the lower Fe₂O₃ content or Gillespie Borate. One more factor: I am using 325 mesh silica here, it dissolves in the melt better.

This dipping glaze has too much raw clay:

So it cracks as it dries

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This is an example of how a dipping glaze that contains too much plastic clay has been applied too thickly. It shrinks and cracks during drying and is guaranteed to crawl. This is raw Alberta Slip. To solve this problem, you need to tune a mix of raw and roasted clay. Enough raw clay is needed to suspend the slurry and dry it to a hard surface, but enough calcine is needed to keep the shrinkage low enough that this cracking does not happen. Perhaps you have been using a glaze having a high percentage of clay, and this does not happen - the reason is likely that the clay is not highly plastic.

When a fluid-melt glaze is applied too thickly it runs

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This is G3806F fired to cone 6 on a porcelain. While you might like the visual effect, note that the thick drips at the bottom. If the thermal expansion is not perfectly matched to the body, the thick gobs will eventually break or fall off.

The glaze on the right is going on way too thick

Switching to ball clay from kaolin fixed it

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Sometimes EP Kaolin is the best suspender in a glaze; sometimes it isn't. These are the same 85% fritted glaze. A (left) employs 15% Old Hickory #5 ball clay to suspend it; B (right) has 15% EPK. B settles quickly, demands low water content, or it runs like water; it goes on very thick even if dipped quickly, it dries instantly and creates uneven thicknesses. By contrast, A goes on like silk, doesn't settle, and dries evenly in about 10 seconds. What a difference! All simply because of using a different clay to suspend it.

A thickly-applied dipping glaze has crawled

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Example of cone 6 glaze crawling on the inside of a stoneware mug. The dipping glaze slurry had a high specific gravity (low water content) and built up thickly on the bisque, even with a quick pour. Thickly applied glazes have more ability to assert their shrinkage and thus commonly compromise their bond with the body during drying. The cracks that appear then become bare patches after firing.

Crawling can happen when paint-on glazes are layered over dipping glazes

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This bowl was dipped in a non-gummed clear dipping glaze. Such glazes are optimized for fast drying and even coverage. However their bond with the bisque is fragile. The blue over-glaze was applied thickly on the rim (so it would run downward during firing). But during drying, it shrunk and pulled the base coat away at the rim (likely forming many tiny cracks at the interface between the clear and the bisque. That initiated the cascade of crawling. When gummed dipping glazes are going to be painted over, a base-coat dipping glaze should be used. What is that? It is simply a regular fast-dry dipping glaze with some CMC gum added (perhaps half the amount as what would be used for painting). There is a cost to this: Longer drying times after dipping and less even coverage. And gum destroys the ability to gel the glaze and make the slurry thixotropic.

Bisque ware of varying wall thickness:

A challenge to glaze evenly

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This is an example of the importance of allowing a bisque piece to dry after glazing the inside surface before glazing the outside face. This hand-built casserole lid is thin and was glazed on the inside first. That wetted the bisque enough that when the outside was poured, there was not enough absorbency remaining to build a sufficient thickness on the darker-colored areas of thinner cross section. The problem is exacerbated by the fact that the underlying red body is darkening the color of the thinner glazed sections.

A glaze thickness tester

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Best on flat surfaces. It rests on three feet. The depth to which a probe in the centre goes (compared to the feet) is displayed on the dial. This is used in industrial settings.

Terra cotta transparent glaze:

Too thick and just right

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When clear-glazing terra cotta ware made from Plainsman L215 special care is needed to get the right thickness of application. This body is a challenge because it contains lots of coarser particles, so if the layer is not thick enough, the glaze surface is rough. But if it is too thick, then suspended micro-bubble clouding mars underglaze decoration and muddies the warm red color of the body.

The mugs are fired at cone 03 to burn out as much of the body LOI as possible and for extra strength. This is G2931K glaze with the C03DRH firing schedule. The mug on the left was double-dipped, there are enough suspended micro-bubbles to lighten the body color and the extra thickness in the handle recesses and thumb-hold have collected them in large sizes. The glaze on the right is not glassy smooth, but represents the best that can be achieved with this body, glaze and temperature combination.

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