Liner-glazing is a very good way to assure that your ware has a durable and leach resistant surface. It also signals customers that you care about this.
Key phrases linking here: liner glaze - Learn more
The term "liner glaze" refers to two things. First, it is a technique (links below), where the inside and outside of a piece have different glazes that meet at the rim. Second, it refers to the practice of choosing a glaze for the inside food surfaces of utilitarian ware based more on its durability and resistance to leaching, running, crawling, blistering and crazing (problems common with reactive glazes). Glossy whites, transparents or modestly colored glazes are most common as liners. Liner glazes can have other practical purposes also. An example is their use in combination with intensely colored or variegated glazes, such are often runny and form a lake in the bottoms of vessels - this can lead to glaze compression failure.
Considerable efforts are required to produce a white or glossy glaze whose fired surface is durable to wear and tear and resistant to leaching and yet works well in the production process. Colored and reactive glazes usually hide imperfections very well, but a transparent glaze makes crazing, micro-bubbles in the matrix, clouding, boron blue, tiny surface dimples and other imperfections plainly visible. Liner glazes can also be colored (if they are well-tested and demonstrably non-leaching). Amber glazes, for example, make more visually appealing liners for dark-burning clay bodies. Colored and opacified liners also hide some imperfections better, this is important if the clay is less processed.
Liner glazes often need to be paired with specific clay bodies (or a family of bodies), primarily because different bodies have different thermal expansions (this is especially so the lower the temperature). Sometimes, the same glaze will work on different bodies, but one or more require a specific firing schedule. The liner should be thermal stress tested to be sure it will stay uncrazed over time (the 300F:Ice-water test for crazing, the Ice-water-to-boiling-water to test shivering).
Matte glazes can also be used as liners, but special effort and expertise are needed to produce a matte surface that is still silky enough to resist cutlery marking and staining. This is easiest to do at higher temperatures. Mattes produced using MgO as a mechanism are the most common.
Liner glazes are used in large quantities and need to have a slurry that stays in suspension. They need to apply to the ware in a smooth even coat, dry quickly and be durable to handling. Until now, in small industry, pottery and hobby pottery circles, clay body manufacturers have left it up to users to formulate their own liner glazes. But increasingly, users are realizing that manufacturers should take on the responsibility to supply reasonably priced pre-mixed powdered/liquid liner glazes for each of their clay bodies (or at least recommend recipes and firing schedules).
Another reason why liner glazes are simple common sense is that you can more easily see when a cup needs to be washed. In addition, although a piece might not be crazed out of the kiln, if crazing happens over time, it is easier to see this glaze in a white or transparent.
Please read the "Base Glaze" glossary page for examples of base glazes for each temperature range.
Liner glazes can be applied in such a way that they meet the outer glaze at the rim (the technique to do this is described in a separate article). Denby Pottery is a company quite skilled at doing this.
This technique enables the use of a liner glaze, in this case GA6-B. These are most valuable where the outer glaze is reactive (melt-mobile, crystallizing or heavily pigmented) and therefore potentially leachable. Not only are liner glazes less likely to leach but they are less likely to craze, this assures water tightness and eliminates any potential for bacteria growth in the cracks (especially if the body has porosity). Liner glazes are also less likely to stain and cutlery mark, adding to the durability of pieces. The straightness of the dividing line is affected by the degree to which the two glazes bleed into each other. Liner glazing also adds a decorative element to pieces. This technique is also practical where mug walls are thin and cannot absorb enough water to dry the glaze quickly after dipping or brushing.
The process of applying a liner glaze to a piece (by pouring), applying wax emulsion and terminating it at the rim, then immersing the outside of the piece into a dipping glaze to apply another color to the outside.
Three cone 6 commercial bottled glazes have been layered. The mug was filled with lemon juice overnight. The white areas indicate leaching has occurred! Why? Glazes need high melt fluidity to produce reactive surfaces like this. While such normally tend to leach metals, supposedly the manufacturers were able to tune the chemistry enough to pass tests. But the overlaps interact, like drug interactions they are new chemistries. Cobalt is clearly leaching. What else? We do not know, these recipes are secret. It is better to make your own transparent or white liner glaze (either as a dipping glaze or brushing glaze). Better to know the recipe to have assurance of adherence to basic recipe limits.
Are commercial glazes really guaranteed food safe? When manufacturers claim adherence to standards like ASTM D-4236 what are they saying? The small amounts of liquid glazes are safe for the artist to brush on. They are not making claims about leaching on finish ware.
Although this is a whiteware body (Plainsman M370), under the transparent liner glaze (G2926B) the color appears ivory, off-white. But with 8% added Zircopax the quality of the color is transformed into a white that is even better than what a much more expensive New Zealand or Grolleg kaolin porcelain would exhibit with a transparent cover glaze. The outer glaze is G2934Y matte with a yellow stain. The glazes were applied inside and outside by pouring and dipping (it would not be possible to apply the yellow evenly enough by brushing).
This is a cone 04 terra cotta piece. The coffee stain cannot be removed because the coffee has also leached off the surface gloss. Glazes are glass. Glass is leachable if the chemistry is out-of-balance. So is this glaze poisoning the user? No, it has an insurance policy. It is transparent, made from a mix of two frits (Ferro 3124, 3134) plus kaolin and silica. The recipe contains no heavy metal colorants or pigments and no toxic fluxes like lithium or barium. But the body is red, how can the glaze be white? A white porcelain-like engobe layer was applied at the leather hard stage and it was clear-glazed after bisque. The fix: The predominant frit, 3134, has almost no Al2O3 (the oxide most important in producing durability). So I increased the Al2O3 (doing the chemistry in my Insight-live.com account). I also began firing one cone higher, at cone 03.
Metal leaching from ceramic glazes: Lab report example
Imagine realizing that pottery you're selling is all going to craze!
Liner Glazing a Stoneware Mug
I will show you how to glaze a mug with a liner glaze inside and a colored one outside so that they meet in a perfect line at the rim.
There is an increasing awareness of the food safety of glazes among potters. Be skeptical of claims of food safety from potters who cannot explain or demonstrate why.
Every glossy ceramic glaze is actually a base transparent with added opacifiers and colorants. So understand how to make a good transparent, then build other glazes on it.
Understand your a glaze and learn how to adjust and improve it. Build others from that. We have bases for low, medium and high fire.
Ceramic glazes are glasses that have been adjusted to work on and with the clay body they are applied to.
How to apply inside and outside glazes to a pottery mug and get them to meet at a clean line at the rim.
300F:Ice Water Crazing Test
Ceramic glazes that do not fit the body often do not craze until later. This progressively stresses the fit until failure point, thus giving it a score
Thermal Shock Failure
A simple test any potter can do by making and firing square tiles and using a plumbing torch to see how long before they fracture.
|By Tony Hansen
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