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Glaze Layering

In hobby ceramics and pottery it is common to layer glazes for visual effects. Using brush-on glazes it is easy. But how to do it with dipping glazes? Or apply brush-ons on to dipped base coats?

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In hobby ceramics (at low temperatures) layering of glazes for decorative effects is commonplace - there are what seem like myriads of choices of bottled glaze products. Even potters are increasingly adopting them for their stoneware pottery and porcelain, suppliers' showrooms are now dominated by bottled brushing glazes. The idea of DIY dipping glazes now seems quaint to many, even though bottled ones are very expensive. Multiple coats are typically applied. Each dries slowly, hardening as it does so (because it contains binders), providing a stable base for the next one. Although the process is slow (compared to dipping) and more difficult to get even coverage, the interactions between layers of varying degrees of opacity, melt fluidity and color produce stunning visual effects - they are an endless source of social media posts.

As noted, commercial bottled glazes contain a binder, typically CMC gum. In contrast, dipping glazes traditionally mixed by potters, are just powdered minerals with a percentage of clay, typically 15-25%, which suspends the slurry and hardens the layer as it dries (and also shrinks it somewhat). These can apply to bisque in the needed thickness with one quick dip and often dry enough to permit handling within seconds. But, their bond with the bisque ware is fragile, especially if the bisque surface is smooth, damp or dense - or the glaze has either excessive or inadequate clay. Still, as long as the first layer is not too thick, the bond with the body is sometimes able to withstand the tension imposed when a second layer rewets and restresses it. But, more often, the second coat pulls the first one off (resulting in peeling or formation of cracks). Glazes recipes that don't pass a sanity check are most likely to fail.

Multi-layering of dipping glazes can work by a balancing act of bisque surface and density, recipe adjustment, slurry preparation and application method. Perhaps you have been doing it with success. This could be because you understand all the variables but also it could be working by accident, a tipping point could be looming, all it might take for sudden failure is a small change one or two factors.

For serious multi-layering of DIY glazes, there is one principle method, as noted above: Adding enough gum to produce a base coat dipping glaze. 1% is a good starting point. Gummed glazes are runnier, they drip (and drip more) and dry slowly on bisque. Traditional potters, used to single-layer gumless dipping glazes, can be shocked at how different it is to work with these. They won’t flocculate, the only practical way to increase viscosity is raising the specific gravity (as high as 1.6). Techniques to apply them evenly must be learned. Unless bisque ware is heated to accelerate drying, patience and ingenuity are needed to get even coverage of adequate thickness. Single dripping points are typically the best drainage option. Of course, spray application is an option, but achieving multilayering will still be time consuming and runs will be a constant issue.

Layering Commercial Bottled Glazes Over a DIY Base Coat

As already noted, the key is to mix the slurry as a base coat dipping glaze, these are gummed, but less so that brushing glazes. If it is then it will tolerate layers of commercial products. All ware needs some sort of cover or base glaze and it makes a lot of sense (especially economic sense) to mix that yourself. You may find that a low percentage of gum will work, that will be a benefit since the glaze will dry faster and go on more evenly.

Firing Issues Regarding Layering of Glazes

-If multiple layers all have high melt fluidity: Each must be thin enough that final conbined thickness will not run excessively.
-A fluid first layer and a non-fluid second: Caution will still be needed as the weight of the second will pull downward on vertical walls.
-A non-fluid layer first and a fluid second layer: Here there is an opportunity to make both of them thicker.
-Two non-fluid layers: Why would you do that? Layering is almost always about making them dance together, interact. And non-fluids will not do that.
-Three layers: Now you are getting adventurous!

Another factor to consider is thermal expansion. Layering a glaze that normally crazes with one that does not could cause cracking when ware exposed to sudden temperature change (because of the internal stresses that would be present).

Food Safety

Commercial glazes are labelled as being food safe, even if the color is very bright (adherence to ASTM D-4236 is actually not very reassuring). Traditional potters are not accustomed to putting high-percentage-heavy-metal glazes on food surfaces. But that is still exactly what many commercial ones are. Use common sense. It is often better to employ non-coloured glazes on food surfaces that must accommodate hot or acidic liquids. When you make your own glazes for these surfaces you know the recipe and you are in control. Consider: Two different drugs may not have serious side effects on their own, but when taken together they can be dangerous. Likewise, a glaze may resist acid attack on its own but you can be sure there exist other glazes, that when mixed with or layered with it, will destabilize it enough to leach metals.

Related Information

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

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.

Commercial brushing glaze on a non-gummed dipping glaze: Crawling

Non-gummed dipping glazes go on evenly and dry quickly on bisque ware (if properly gelled). But they only work well as a single layer. If you try to paint commercial gummed brushing glazes over them the latter will compromise their bond with the body, cracks will develop during drying and bare patches like this will result during firing. For multi-layering the base dipping glaze must be gummed (e.g. 1% CMC gum). It will go on thinner, drip longer and dry much slower, but that is the price to pay if you want to layer over it.

Multi-Layering on a large bowl has produced crawling

Glaze layering is tricky. We recommend a gummed base coat dipping glaze where layering will be done over it. This alone could have solved this issue.

Pottery glaze peeling when multi-layered

Dipping glazes can, in very controlled circumstances, be multi-layered. If you have done it for some time, with success, you may have been just lucky. These pieces demonstrate one of many factors that can produce failure: The top glaze contains 7% bentonite and 5% zinc oxide - that is 12% hyper-fine particles, perfect to create the drying shrinkage to make this happen. The recipe author must have reasoned that it could "pinch hit" for the inadequate clay content. But 7% bentonite in any glaze is highly unusual. And, it is actually not even necessary here. Why? The high percentage of Ferro Frit 3124 is sourcing needless Al₂O₃ (alumina), that should be coming from kaolin or ball clay instead. Frit 3134 is the perfect stand-in, it contains almost no Al₂O₃, but otherwise is quite similar. The equivalent recipe we calculated on the right has the same chemistry, but does pass a sanity check. It is not guaranteed to work, but has a better chance than this one. For assurance of success, the base should be mixed as a base-coat dipping glaze.

Variegating effect of sprayed-on layer of titanium dioxide

The base glaze (inside and out) is GA6-D Alberta Slip glaze fired at cone 6 on a buff stoneware. However, on the outside the dried-on glaze was over-sprayed with a very thin layer of titanium and water (VeeGum can be used to help gel the sprayable titanium slurry and suspend it). The dramatic effect is a real testament to the variegating power of TiO2. An advantage of this technique is the source: Titanium dioxide. It is a more consistent source of TiO2 than the often-troublesome rutile. Another advantage is that the variegation can be selectively applied in specific areas or as a design. This effect should work on most glossy glazes having adequate melt fluidity.

Cone 6 black with a second layer of oatmeal glaze

The underglaze is G1214M cone 6 black (adds 5% Mason 6666 black stain). Overglaze left: GR6-H Ravenscrag Oatmeal. Overglaze right: GA6-F Alberta Slip oatmeal. Both produce a very pleasant silky matte texture (the right being the best). Both layers are fairly thin. In production it would be best to spray the second layer, keeping it as thin as possible. It is also necessary to adjust the ratio of raw to roasted Alberta or Ravenscrag Slips to establish a balance between drying hardness but not too much drying shrinkage (and resultant cracking).

Ravenscrag oatmeal layered over black at cone 6

This is GR6-H Ravenscrag Slip oatmeal over G1214M black on porcelain at cone 6 oxidation to create an oil-spot effect. Both were dipped quickly.

Layering glazes to get variegation

Example of the variegation produced by layering a white glaze of stiffer melt (a matte) over a darker glaze of more fluid melt (a glossy). This was fired at cone 6. The body is a stoneware and the glazes employ calcium carbonate to encourage bubbling during melting, each bubble reveals the color and texture of the underlying glaze layer. It is also possible to get this effect using the same base glaze (stained different colors).

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