How many layers of glaze do you need?

Typically, three coats are applied. Each dries slowly, hardening as it does so (the glazes contain binders). This provides a stable base for the next one.

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All Glossary

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?

Details

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 stoneware potters are increasingly adopting bottled brush-on glazes for their stoneware pottery and porcelain, suppliers showrooms are now dominated by bottled glazes. The idea of mixing your own recipe and dipping ware now seems quaint to many potters. This is in spite of the fact that bottled glazes are very expensive. Typically, three coats are applied. Each dries slowly, hardening as it does so (the glazes contain binders). This provides a stable base for the next one. Although the process is slow (compared to dipping) and it is more difficult to get even coverage, the interactions between layers of varying degrees of opacity, melt fluidity and especially color can produce stunning visual effects. As noted, commercial bottled glazes contain a binder, typically CMC gum. Why? Consider traditional dipping glazes. They are intended as single-layer. They are just powdered minerals with a percentage of clay, typically 15-25%, it suspends the slurry and hardens the layer as it dries. Dipping glazes can apply to bisque in the needed thickness with one quick dip 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 denser or the bisque is damp. Still, as long as the layer is not too thick, the bond is able to withstand the tension imposed by the shrinkage during drying (without cracks forming). But, the traditional dipping glazes normally can only be layered under special conditions. If the bisque is absorbent, the first layer is still damp (not wet) and it is not too thick a second dipped coat may be tolerated. Likewise, control of the raw clay content can help make multi-layering possible (e.g. if a recipe contains 30% raw kaolin, 15% calcined and 15% raw would reduce the drying shrinkage, if a recipe lacks clay (and therefore is not bonding well with the bisque) sourcing some of the Al 2 O 3 from kaolin instead of feldspar will bond it better (you need glaze chemistry to do that). But for serious multi-layering of glazes that you mix yourself, there is one principle method: Adding gum to the recipe. 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 dipping glazes, can be shocked at how different it is to work with gummed glazes. They won’t flocculate, the only practical way to make them thicker is raising the specific gravity (as high as 1.6, vs. dipping which can be as low as 1.4). You need to learn ways to apply them evenly and quickly. More absorbent bisque helps (e.g. fired to cone 06). Even heating thinner ware before dipping will greatly accelerate drying speed. Ware needs to be designed to be able to drain to a single point after dipping (since it is often necessary to hold a piece upside down for a minute while waiting for dripping to stop). Of course, spray application is also an option, but multilayering will likely still be necessary. And runs will be a constant issue. As noted above, your glaze needs to be gummed (e.g. 1%). 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 fast and go on more evenly.

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Firing Issues Regarding Layering of Glazes

-If multiple layers all 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 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 labeled as being food safe, even if the color is very bright. Traditional potters are not accustomed to putting high-percentage-heavy-metal glazes on food surfaces. But that is 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

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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.

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

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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.

Multi-Layering on a large bowl has produced crawling

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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.

Layering glazes to get variegation

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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).

Ravenscrag oatmeal layered over black at cone 6

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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

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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).

Variegating effect of sprayed-on layer of titanium dioxide

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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 glaze was over-sprayed with a very thin layer of titanium. 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.

Learn to mix any of your glazes for these three application methods

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Potters are used to making dipping glazes that they weigh out and mix from recipes. Hobbyists commonly use bottled commercial brushing glazes. Did you know that a dipping glaze can be turned into a brushing glaze by the addition of Veegum (or Veegum CER) and water? Do you know what a base-coat dipping glaze is? Here is a quick overview: Dipping glazes need to go on to bisque ware evenly, be thixotropic enough to hold on at thickness and drain and dry quickly. But they don't need to dry hard. Brushing glazes need a cohesive slurry that dries slowly and hardens well on drying. They also must adhere to the body really well so that multiple layers can be applied (individual layers go on thin). Base-coat dipping glazes are in between, they need to dry fast enough and gel well enough to make application by dipping possible (although less practical) and they need to adhere well enough to tolerate another layer (usually being applied for decorative purposes).

How many layers of glaze do you need?

Which cocktail is lowest in calories?

Which diet soda is the healthiest?

Traditional “juice” activates 24/7 fat-burning

All Glossary

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?

Details

Does insurance cover colon cleanse?

What 2 foods should not be eaten together?

Strange purple cocktail removes 2.7lbs every 4 days

Firing Issues Regarding Layering of Glazes

-Three layers: Now you are getting adventurous!

Food Safety

Related Information

Tap picture for full size

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

Tap picture for full size

Multi-Layering on a large bowl has produced crawling

Which is better smoothies or juicing?

What foods remove toxins from your body?

Strange purple cocktail removes 2.7lbs every 4 days

Tap picture for full size

Layering glazes to get variegation

Tap picture for full size

Ravenscrag oatmeal layered over black at cone 6

Tap picture for full size

Cone 6 black with a second layer of oatmeal glaze

Tap picture for full size

Variegating effect of sprayed-on layer of titanium dioxide

Tap picture for full size

Learn to mix any of your glazes for these three application methods

Tap picture for full size

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