In hobby ceramics (at low temperatures), layering of glazes for decorative effects has been commonplace for many decades. Potters have traditionally used dipping and pouring techniques, but in recent years they have increasingly adopted commercial prepared brush-on glazes for their stoneware pottery and porcelain. As with low fire, these high temperature prepared glazes have added gum to make them paintable. Typically, three coats are needed to achieve a thick enough layer. Each layer dries slowly, but hardens as it does so, providing a stable base for the next one. If the melt fluidity is not too high, more than three coats can be tolerated. Although this process is slow (compared to dipping) and it is more difficult to get even coverage, the opportunity to layer glazes of different colors and characters to produce reactive visual effects has become highly appealing to many potters. These effects are a product of interactions between layers of varying degrees of opacity and melt fluidity. And at stoneware temperatures, more materials, melting and crystallization mechanisms are available (compared to low fire) to produce really interesting visuals. All this being said, even gummed glazes can experience difficulty in adhering to smooth surfaces (porcelain) or ware that has been bisque fired high enough to make it dense.
Ravenscrag oatmeal layered over black at cone 6
This is GR6-H Ravenscrag oatmeal over G1214M black on porcelain at cone 6 oxidation to create an oil-spot effect. Both were dipped quickly. You can find more detail at ravenscrag.com.
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 calcined Alberta or Ravenscrag Slips to establish a balance between drying hardness but not too much drying shrinkage (and resultant cracking).
Two glazes. One crawls, the other does not. Why?
The glaze on the right is crawling at the inside corner. Why? Multiple factors contribute. The angle between the wall and base is sharper. A thicker layer of glaze has collected there (the thicker it is the more power it has to impose a crack as it shrinks during drying). It also shrinks more during drying because it has a higher water content. But the leading cause: Its high raw clay content increases drying shrinkage. Calcining part of the raw clay destroys its affinity for water (which is what makes it plastic), this is an effective way to deal with this. Or doing a little chemistry to source some of the Al2O3 from materials other than clay (e.g. a frit having a higher Al2O3 content).
Variegating effect of sprayed-on layer of 100% titanium dioxide
The referred to surface is the outside of this large bowl. The base glaze (inside and out) is GA6-D Alberta Slip glaze fired at cone 6 on a buff stoneware. The thinness of the rutile needs to be controlled carefully, the only practical method to apply it is by spraying. The dramatical effect is a real testament to the variegating power of TiO2. An advantage of this technique is the source: Titanium dioxide instead of sourcing TiO2 from the often troublesome rutile.
Why you should not paint pure stain powders over glaze
On the left is a pure blue stain, on the right a green one. Obviously, the green is much more refractory. On the other hand, the green just sits on the surface as a dry, unmelted layer. For this type of work, stains need to be mixed into a glaze-like recipe of compatible chemistry (a medium) to create a good, paintable color. The blue is powerful, it would only need to comprise 5-10% of the recipe total. Its medium would need to have a stiffer melt (so the cobalt fluxes it to the desired degree of melt fluidity). A higher percentage of the green stain is needed, perhaps double. It's medium needs much more melt fluidity since the stain is refractory. Of course, only repeated testing would get them just right. Guidelines of the stain manufacturer for chemistry compatibility need to be consulted also (as certain stains will not develop their color unless their glaze medium host has a compatible chemistry). And, to be as paintable as possible, use use a gum-solution/water mix (e.g. 2 parts water to one part gum solution).
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.
When glazes are highly fluid they can...
An example of a highly fluid glaze melt that has pooled in the bottom of a bowl. The fluidity is partly a product of high KNaO, thus it is also crazed (because KNaO has a very high thermal expansion). While it may to decorative, this effect comes at a cost. The crazing weakens the piece, much more than you might think (200%+). Those cracks in that thick layer at the bottom are deep, they want to continue down into the body and will do so at the first opportunity (e.g. sudden temperature change, bump). Also, fluid glazes like these are more likely to leach.
Dip-glazing vs. brush-on glazing: Which gives the more even surface?
This is a clear glaze (G2931K) with 10% purple stain (Mason 6385). The mugs are cone 03 porcelain (Zero3). The mug on the left was dipped (at the bisque stage) into a slurry of the glaze (having an appropriate specific gravity and thixotropy). The glaze dried in seconds. The one on the right was painted on (two layers). Like any paint-on glaze, it contains 1% CMC Gum. Each layer required several minutes of application time and fifteen minutes of drying time.
Gum content in glazes can have a dramatic effect on drying time
This is a low fire brushing glaze. It has been sitting on this plaster bat for two hours and shows little sign of dewatering. A typical pottery dipping glaze, by contrast, would dewater in seconds! Clearly, such glazes are only good for brushing.
Common dipping glazes converted to jars of brushing glazes
These are cone 6 Alberta Slip recipes that have been brushed onto the outsides of these mugs (three coats). Recipes are GA6C Rutile Blue on the outside of the left mug, GA6F Alberta Slip Oatmeal on the outside of the center mug and GA6F Oatmeal over G2926B black on the outside of the right mug). One-pint jars were made using 500g of glaze powder, 75g of Laguna CMC gum solution (equivalent to 1 gram gum per 100 glaze powder) and 280g of water. Using a good mixer you can produce a silky smooth slurry of 1.6 specific gravity, it works just like the commercial bottled glazes. The presence of the gum makes it unnecessary to calcine the Alberta Slip.
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