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Glaze Layering


Glazes are often layered for decorative effects. These effects are a product of interaction between the layers or varying degrees of opacity in upper layers (that reveal artifacts of those below). However a number of issues can arise with this process and understand the causes will certainly give you more control and success.

Glazes, when used alone in a single layer, almost always have an ideal thickness. Thinner than that and they look washed out, do not properly cover texture on the body surface or exhibit the wrong color. Thicker than that and they often run downward as they melt, gluing ware to the kiln shelf (or creating unsightly globs at the base). Thicker glazes have another more serious issue: they can form cracks during drying (especially if they have higher clay or water content). That causes crawling. This happens because glazes have a rather fragile bonding mechanism with the bisque-ware (or dry-ware) surface. As long as the glaze layer is not too thick, and the recipe has enough clay to make it dry hard, the bond is able to withstand the tension imposed by the shrinkage without cracks forming. But when the glaze layer is thicker it is empowered to literally pull itself away from the body as it dries (and forms a "dried up lake bed" kind of surface). Applying double or triple layers can completely mess up this whole thing! So how can one do it?

The term 'glaze' can mean many things in ceramics and pottery. Underglazes are almost always layered (at a minimum where they overlap). However underglazes do not melt like regular glazes, they are formulated to melt only enough to form a bond with the fired body. They vitrify, rather like a porcelain (at least they should but unfortunately some do not melt as much as they should and others melt too much, choose your brand carefully). But they have something else a glaze does not have: Gum. A binder. They are loaded with glue, that is why they dry so slowly and harden really well. They harden and bond so well than you can paint them on to leather hard ware (in fact this is often the preferred method). This gum and their fired stability make it possible to layer underglazes as much as you want. A clear overglaze is almost always applied over the underglaze layers.

How does one layer regular glazes? Logic. Add CMC gum to all your glazes. But the problem with that is they can literally take ten times as long to dry and they drip, drip, drip. This slows down the dipping process tremendously. A glaze recipe with 15-20% kaolin or ball clay will often dry in seconds with no drips on bisque ware (if the glaze has the right specific gravity and thixotropy). That is so much better. When glazes have more clay than this they have more shrinkage and 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). 15-20% is not a hard-fast rule, maybe your need to go lower (by calcining, see below).

When do you put on the second layer? This depends on the nature of the first layer. If it is very thin and quite hard you should be able to do layer two right away. But if the first layer is taking a long time to dry (e.g. on thin walled bisque ware), then you will need to wait. Sometimes it is better not to wait until the first layer (and the bisque) are completely dry since re-wetting of that first layer can compromise the bond. As long as the first layer still has a little moisture in it it will be much more impermeable to being re-wetted.

Consider some scenarios:

-You are putting on two layers and both glazes have good melt fluidity: Each layer will need to be half as thick as normal. Or you will need to modify ware contour to be able to tolerate more running.
-You are putting 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.
-You are putting a non-fluid layer first and a fluid second: Here you have opportunity to make both of them thicker.
-You are putting 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. If you layer together a glaze that normally tends to craze with one that does not you are asking for trouble if it is functional ware. What trouble? Cracking when exposed to sudden temperature change.

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

What if it is not working? Then you need to do more calcining to reduce the plastic clay content. Or heat ware before applying each layer. Or add gum and paint them on. Or apply by spraying instead of dipping. Or gumming and then spraying. 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.

Pictures

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. 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

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.

Double-slip layer incised decoration: A challenge in slip-body fitting

An example of a white engobe (L3685T) applied over a red clay body (L3724F), then a red engobe (also L3724F) applied over the white. The incised design reveals the white inter-layer. This is a tricky procedure, you have to make sure the two slips are well fitted to the body (and each other), having a compatible drying shrinkage, firing shrinkage, thermal expansion and quartz inversion behavior. This is much more complex that for glazes, they have no firing shrinkage and drying shrinkage only needs to be low enough for bisque application. Glazes also do not have quartz inversion issues.

Why you cannot paint pure stain powders over glaze

An example of why you should not just paint pure stain powders over glazes. Left is a blue stain, right is a green. Obviously the blue is melting in much better, even bleeding at its edges. On the other hand, the green just sits on the surface as a dry, unmelted layer. Stains need to be mixed into a glaze-like recipe of compatible chemistry (a melt medium). The blue is powerful, it would only need to comprise 5-10% of the total, the green 10%-15%. Overglaze recipe development projects involve following the guidelines of the stain manufacturer for chemistry compatibility and adjusting the melt to compensate for each stains melting behavior.

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.

Out Bound Links

  • (Glossary) Crawling

    A condition where glaze separates into clumps or i...

  • (Glossary) Specific gravity

    A comparison of the weights of equal volumes of a ...

  • (Glossary) Thixotropy

    Knowing about thixotropy will enable you to mix a ...

  • (Glossary) Calcine, Calcination

    The calcining process is used to remove some or al...


By Tony Hansen




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