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Glaze Layering


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.

Dipping glazes, by contrast, have a whole different dynamic. They typically contain no gum, they are just powdered minerals with a percentage of clay (it suspends the slurry and hardens the layer as it dries). Dipping glazes do not need to be multilayered to go on thick enough. When a slurry is prepared properly, a 2-second dip is usually enough to get adequate thickness. And they can dry enough to permit handling within seconds. But, their bond with the bisque ware is fragile, especially if the surface is smoother or more dense. As long as the glaze layer is not too thick, the recipe has enough clay to make it dry and not too much to make it shrink excessively, the bond is able to withstand the tension imposed by the shrinkage during drying (without cracks forming). But when the glaze layer is thicker it is empowered to literally pull itself away from the body as it dries (sometimes forming a "dried up lake bed" kind of surface). As you can imagine, trying to apply multiple layers by dipping is a tricky process indeed. But it is possible.

A dipping, non-gummed glaze recipe with 15-20% kaolin or ball clay will often dry in seconds with no drips on sufficiently porous bisque ware (if the glaze has the right specific gravity and thixotropy). If a layer is thin enough a second dipped one can often be tolerated. However, when glazes have a higher clay percentage they have more shrinkage, that is bad for multi-layering. You can fix this by calcining some of the clay, that reduces the drying shrinkage (and also the hardness). For example, if a recipe contains 30% raw kaolin, it would be beneficial to use 15% calcined and 15% raw kaolin (actually the calcined portion should be 12% less since the LOI of the raw kaolin is 12%).

Usually it is best if the first layer is still damp when applying a second one (re-wetting of that first layer is more likely to compromise the bond with the body if it is bone dry). If the first layer is taking a long time to dry (e.g. on thin walled bisque ware), then you need to wait.

There are also things to consider regarding the melting properties of glazes to be layered. Consider some scenarios:

-Two layers and both glazes have high melt fluidity: Each layer will need to be thinner than normal. Or contours will need to be modified to be able to tolerate more running.
-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 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.

Another factor to consider is thermal expansion. Layering a glaze that normally crazes with one that does is asking for trouble if it is functional ware. What trouble? Cracking when exposed to sudden temperature change (because of the internal stresses that would be present).

What about a third layer: All the same logic applies.

What if the layering is not working? Try calcining to reduce the plastic clay content. Or heat ware before applying each layer. Or add gum and paint them on. Or applying by spraying. Or gumming the first layer. Or even bisquing on layers before adding others. Or reformulating low-clay glazes to source some of the Al2O3 from kaolin instead of feldspar (done by chemistry). The objective is to get layers of hardened powder of the right thickness stuck on to the surface of the ware well enough to fire without crawling. Whatever it takes to physically make that happen is what you have to do if you want to layer specific glazes.

Ravenscrag oatmeal layered over black at cone 6

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

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?

Two glazes. One crawls, the other does not. Why?

The glaze on the right is crawling at the inside corner. 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 is an effective way to deal with this. Or doing a little chemistry to source some Al2O3 from other materials than clay.

Variegating effect of sprayed-on layer of 100% titanium dioxide

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

Why you should not paint pure stain powders over glaze

On the left is a blue stain, right is a green. Obviously the blue is melting much better, even bleeding at its edges. 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 melt 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). The percentage of the green stain would need to be higher (10%-15% or more). It's medium would need to be fluid (over melted), the stain would then stiffen it up to give desired melt fluidity. Of course, only repeated testing would get them just right. The guidelines of the stain manufacturer for chemistry compatibility would need to be consulted also (as certain stains will not develop their color unless the medium they are in has a compatible chemistry). And, to be as paintable as possible, use 1 part of gum solution to each 2 parts of water to create the slurry.

Layering glazes to get variegation

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...

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?

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

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

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.

Out Bound Links

  • (Glossary) Crawling

    A condition where glaze separates into clumps or islands (during firing) leaving bare clay patches showing in-between. This problem is almost always caused by glazes shrinking too much during drying and then cracking. Those cracks become the crawl-points during firing. Excessive shrinkage is norm...

  • (Glossary) Specific gravity

    A comparison of the weights of equal volumes of a given liquid and water. A ceramic slurry with a specific gravity of 1.8 is thus 1.8 times heavier than water. The best way to measure specific gravity is to weigh a container and record its weight, then weigh the container full of water and full of t...

  • (Glossary) Thixotropy

    Knowing about thixotropy will enable you to mix a glaze that stays in suspension much better. It does not drip alot when a piece is dipped into it. It goes on evenly and does not run. It dries quickly (on porous bisque) and is just much nicer to use. The secret to all of this is not intuitive. It in...

  • (Glossary) Calcine, Calcination

    The calcining process is used to remove some or all unwanted volatiles from a material (e.g. H2O, CO2, SO2) and/or to convert a material into a more stable, durable or harder state. Varying temperatures are employed to calcine various materials, depending on the decomposition temperature of the vol...

  • (Glossary) Brushing Glazes

    In North America, the hobby pottery community has transformed in recent years toward the use of commercially prepared, gummed, paint-on glazes. Almost all potters in the past made their own glazes but now many have embraced the use of small bottles of paint-on glazes for some or all of their ware. W...


By Tony Hansen




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