Liner-glazing ceramic ware 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.
It is common to glaze food surfaces of utilitarian ware with a brilliant glossy white or transparent glaze called a 'liner glaze'. This is done to avoid releasing in-glaze or on-glaze metallic colorants to food or drink (which could leach them away and be a health hazard). Liner glazes can be applied in such a way that they meet the outer glaze at time rim (the technique to do this is described in a separate article). Denby China is a company quite skilled at doing this. It is common to simply add colorants, opacifiers and variegators to liner glazes to produce the decorative glazes using on the outsides are ware.
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 do 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 is white or transparent.
Please read the "Base Glaze" glossary page for examples of base glazes for each temperature range.
An example of how a liner glaze can meet another at the rim of a piece. This it quite simple to do. The technique is especially practical where mug walls are thin and cannot absorb enough water to dry the glaze after immerse-dipping. It is essential where the outer glaze is potentially leachable, or it might craze (which tenmokus often do). Thus, that straight line at the rim is not only a decorative element, it is the spot where leaching, crazing, staining and cutlery marking stop.
Notice they have a liner glaze. The outside decoration has been done using wax resist and transfer techniques.
The white inner glaze (which wraps over the rim) was not removed after the wax was applied. The outer glaze thus overlays it near the rim. The meeting line has been blurred and the other glaze has run downward and crawled somewhat revealing patches of the inner one.
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, it is 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 was applied at the leather hard stage and it was clear-glazed after bisque. The fix: The predominant frit, 3134, has almost no Al2O3. So I increased it (doing the chemistry in my Insight-live.com account) and began firing at cone 03.
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 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).
Three cone 6 commercial bottled glazes have been layered. The mug was filled with lemon juice over night. The white areas on the blue and rust areas on the brown have leached! Why? Glazes need high melt fluidity to produce reactive surfaces like this. While such are normally subject to leaching, the manufacturers were able to tune the chemistry of each to make them resistant. But the overlaps mingle well (because of the fluidity), they are new chemistries, less stable ones. What is leaching? Cobalt! Not good. What else? We do not know, these recipes are secret. It is much better to make your own transparent or white liner glaze. Not only can you pour-apply it and get very even coverage, but you know the recipe, have control, can adjust to fit your body.
This lab is certified by the US Department of Environmental Protection (DEP) for drinking water and waste water analysis. They also provide pottery glaze leaching analyses (the water is kept in contact with the glaze then analysed for trace levels of specific metals). Each suspected metal to be tested for entails a separate charge ($30-60 in this case). That means that testing one glaze for several metals could cost $200. How to make sense of these numbers? Google the term: "heavy metals drinking water standards", and click "Images" to find charts with lots of data. Searching pages for this term will find books having detailed sections on each of the metals. Typically you are only interested on one metal in a specific glaze (often cobalt or manganese). There are ways to sleep better (about the likelihood your glazes are leaching metals) if you cannot do this: Do a simple GLLE test. And avoid the online trafficking in hazardous recipes. Better to find a quality base glaze (matte and transparent) that works well on your clay body. Then add colorants, opacifiers and variegators; but doing so in a conservative manner.
Understanding your transparent glaze and learning how to adjust its melt fluidity, thermal expansion, color response, etc is a base on which to build all your other glazes.
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.
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.
|Tests||Thermal Shock Failure|
|Tests||300F:Ice Water Crazing Test|
|Media||Liner Glazing a Stoneware Mug|
How to Liner-Glaze a Mug
A step-by-step process to put a liner glaze in a mug that meets in a perfect line with the outside glaze at the rim.