|Monthly Tech-Tip |
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.
Potters soon learn the need to concern themselves with the thickness of glazes they apply to ware, each one having its own requirement. This involves selection of bisque temperature (to control bisque porosity and thus the speed of thickness buildup); duration of dipping or number of brush coats applied; the viscosity, thixotropy and specific gravity of the slurry; the percentage of plastic clays in the slurry, the temperature and wetness of the bisque. On some types of ware (e.g. transparently glazed fine porcelain) the glaze must be thin and 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 - potter's 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 (which generate gases of decomposition during firing). Fluid melt glazes, or those having high surface tension at melt stage, can blister on firing if applied too thick. Glazes having sufficient clay to produce excessive shrinkage on drying will crack (and crawl during firing) if applied too thick. Fluid melt glazes will run off ware 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 will often craze if applied too thick. Transparent colored glazes will fire the wrong shade if not the right thickness. 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 thickness of glaze application. Reactive glazes that variegate or crystallize usually require a specific thickness (and a specific firing curve). Crystalline glazes must be applied too thick so they will run during firing. Many types of ware (e.g. functional porcelain and stoneware) require that the glaze 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.
With all these variables it is thus mostly experience that will help you determine the right thickness for a glaze. Although there are thickness-measuring devices, in most cases a trained eye can tell, just by looking at a fresh application, if it is right (credit-card-thickness is about right for most common glaze types, if not they can be gauged in relation to that). If the specific gravity and thixotropy of glaze slurries are maintained, the same type of clay is used, the bisque firing is always done at the same temperature and the type of ware is consistent, technicians tend to gauge thickness simply by the dipping time.
This is an example of how a glaze that contains too much plastic clay has been applied too thick. 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 calcine material. Enough raw 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. The Alberta Slip website has a page about how to do the calcining.
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, dries evenly in about 10 seconds. What a difference! All simply because of using a different clay to suspend it.
Two transparent glazes applied thickly and fired to cone 03 on a terra cotta body. Right: A commercial bottled clear, I had to paint it on in layers, I ended up getting it on pretty thick. Left: G1916S, a mix of Ferro frits, nepheline syenite and kaolin - one dip for 2 seconds and it was glazed. And it went on more evenly. Bubbles are of course generated by the body during firing. But also in the glaze. Raw kaolin loses 12% of its weight on firing, that produces gas. Low temperature glazes melt early, while gassing may still be happening. Keeping raw clay content in a glaze as low as possible is good, but at least 15% is normally needed for working properties. Improvements? Both of these could have been applied thinner. And I could have fired them using a drop-and-hold and a slow-cool schedule.
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 mug on the left has three coats of Spectrum Majolica base, painted on by brush. 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 not even at all! The one on the right was dipped in a 5 gallon bucket-full of G3890 Arbuckle 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 making your own 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).
Why? Glaze fit. These are available on Aliexpress (as Drip Pottery) and they are made by a manufacturer that has close control of body maturity (and thus strength) and a dilatometer to precisely match the thermal expansion of the glaze. The glaze has to fit better than normal because of the absence of an outside glaze. Too low an expansion and it's compression (outward pressure) will fracture body (because these are thin-walled pieces). Too high and it will craze. And that thick glaze? It will shiver or craze with far less forgiveness than a thin layer. And how did they get the glaze on this thick? They deflocculated it, up to 1.7 or more, glazed the inside, let it dry, then glazed the outside. These pieces are a visual and technical achievement. If you are a potter you had best think twice before attempting the same.
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.
Example of glaze crawling on the inside of a stoneware mug. Notice how thick it is. Thickly applied glazes have more ability to assert their shrinkage during drying and thus compromise their bond with the body below. The cracks that appear become bare patches after firing.
This is G2415J Alberta Slip glaze on porcelain at cone 6. Why did the one on the right crawl? Left: thinnest application. Middle: thicker. Right thicker yet and crawling. All of these use a 50:50 calcine:raw mix of Alberta Slip in the recipe. While that appears fine for the two on the left, more calcine is needed to reduce shrinkage for the glaze on the right (perhaps 60:40 calcine:raw). This is a good demonstration of the need to adjust raw clay content for any glaze that tends to crack on drying. Albertaslip.com and Ravenscrag.com both have pages about how to calcine and calculate how much to use to tune the recipe to be perfect.
The body is Plainsman L215. Both were thinly applied and fired using the 04DSDH schedule. The glaze has 2% iron oxide added and sieved to 80 mesh, this reddens the color and acts as a fining agent to reduce micro-bubble population. The one fired to cone 03 (left) is considerably stronger, better surviving the stress of successive impacts with a hammer. However, it has minute dimples in the surface, likely because it is having to clear bubbles originating from decompositions occurring in the body below (as it fires well above bisque temperature). The mug on the right fired about 40F cooler, between cone 04 and 05, only slightly above bisque. The glaze surface is much better, almost crystal clear. Since the glaze fits well the mug has surprising strength, much better than a stoneware piece with a poorly fitted glaze that shatters with one tap of a hammer. This one survived about ten whacks before a piece broke out! A big advantage of cone 04 and cooler is that ware can be fired on stilts, meaning you can glaze the whole thing, no bare clay is exposed. Again, I can only achieve this kind of glaze surface using the above-mentioned firing schedule.
This bisque mug has been glazed on the inside. But the bisque has absorbed water from that glaze and this thin walled mug is now water logged as a result (except at the thicker base). It does not have the absorbency needed to build up a thick enough layer of glaze on the outside. Even if it did, the water from the two glazes would wet the bisque so much that its drying time would be greatly extended. This is a problem because the mechanism of attachment of glaze to body is fragile and works best when the glazes dries quickly (if drying is too slow bubbling and cracking can result).
Left: G2934 cone 6 matte glaze with 3% Mason 6300 blue stain. Right: An additional 4% tin added. The thickness of application is close to the same. Notice how an opacified color does not have depth and therefore is lighter in color. Also it does not break to different shades at the edges of contours the way the transparent color does.
This is the Ravenscrag slip cone 6 base (GR6-A which is 80 Ravenscrag, 20 Frit 3134) with 10% Mason 6006 stain. 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, this transparent base has more entrained micro-bubbles than a frit-based glaze, these enhance the color effect.
This is an example of the importance of allowing a bisque piece to dry after glaze the inside surface before glazing the outside face. This hand-built caserole 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.