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.
There are many good reasons to use a liner glaze for your mugs. It may appear to take extra effort at first, but once the technique becomes a part of your standard practice you will find it has many advantages. Why?
Of course, the biggest challenge of using a liner glaze is getting the interior and exterior glazes to meet in a clean line. But it is actually quite easy.
As mentioned, for all this work (and be worth it) you need to have a good functional and easy-to-use liner glaze. There is no reason to put up with problems of excessive dripping, settling in the bucket, crazing, slow drying, running or other things. If you are having a few problems with the glaze you have do not just throw it out and search for another, fix it. How?
Left: Ravenscrag G2928C matte on inside of mug. Right: A clear glossy. The matte needs to be soaked in the kiln long enough to make sure it develops a functional surface, especially on the bottom. Mattes are not always the best choice for food surfaces, but you can do it if you blend in enough glossy glaze to make it smooth enough not to cutlery mark.
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.
The liner is G2571A dolomite matte.
This is the G2571A glaze recipe. It has proven reliable and functional over many years on a wide range of clay bodies in the Plainsman Clays studio. Actually, a better brown color can be achieved using manganese dioxide.
The mug on the left was bisque fired and then glazed, the one on the right was glazed in the green (dry) state. The glazes are the same inside and out but the porcelain one the right is based on New Zealand kaolin (vs. American kaolin on the left). Three secrets for success for the one on the right were: It was glazed inside and out in two operations with a drying phase between, it was heated to about 150F before each application and it was fired with a soaking period (at about 1900F) on the way up to top temperature (cone 6).
The mug on the right was bisque fired and then glazed, the one on the left was glazed in the green (dry) state using our standard meet-two-colors-at-the-rim glazing method. This method lends itself well to single fire glazing. Notice the glaze did not go on as thick on the once-fired piece (extra attention is needed to make sure it gets on thick enough without cracking the piece). In addition, there are a few pinholes whereas the bisqued piece is flawless. Single firing ware requires extra attention to firing, climbing to a point just before the glaze begins to melt and soaking there to enable hydrates and carbon to escape.
With the proper technique you can make the outside and inside glazes meet in a straight line at the rim rather than just have one spill over the other.
If your drying glaze is doing what you see on the left, do not smooth it with your finger and hope for the best. It is going to crawl during firing. Wash it off, dry the ware and change your glaze or process. This is Ravenscrag Slip being used pure as a glaze, it is shrinking too much so I simply add some calcined material to the bucket. That reduces the shrinkage and therefore the cracking (trade some of the kaolin in your glaze for calcined kaolin to do the same thing). Glazes need clay to suspend and harden them, but if your glaze has 20%+ kaolin and also bentonite, drop the bentonite (not needed). Other causes: Double-layering. Putting it on too thick. May be flocculating (high water content). Slow drying (try bisquing lower, heating before dipping; or glaze inside, dry it, then glaze outside).
The outer green glaze on these cone 6 porcelain mugs has a high melt fluidity. The liner glaze on the lower one, G2926B, is high gloss but not highly melt fluid. Notice that it forms a fairly crisp boundary with the outer glaze at the lip of the mug. The upper liner is G3806C, a fluid melt high gloss clear. The outer and inner glazes bleed together completely forming a very fuzzy boundary.
The color is developing despite the fact that very little iron is available from the body. I have glazed the inside of this mug with a durable liner glaze to make it functional. The porcelain contains more than 30% silica but the Shino is still crazing on it.
B-Mix is a popular high-ball clay very plastic grey cone 10R stoneware in North America. The two mugs on the left have pure Ravenscrag Slip on the inside (the middle on the outside also), it fires almost transparent with a slightly silky surface. Pure Alberta Slip is employed on the outside of the left one and the inside of the right one. The outside of the right one is RavenTalc silky matte. In all cases the Ravenscrag and Alberta Slip are mixed half-and-half calcined and raw. B-Mix fires dark enough and with enough specks that a normal transparent glaze is not very interesting. But these Ravenscrag ones look much better (for liner glazes).
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.
Left is Plainsman M340. Right is M390. Each mug has been white engobed inside and half-way down the outside. The insides have been glazed using G2926B clear. The inside surface has more depth and has a richer appearance than you could achieve using a white glaze (especially over the dark burning body). The outside of the left one is Alberta Slip base GA6A using Frit 3195 (it produces a more stable glass of lower thermal expansion). The outside glaze on the right is the clear plus 4% iron oxide. This technique of using the engobe enables porcelain-like functional surfaces on the insides and striking visual contrast and character on the outside of the dark body mug.
Black burning bodies are popular with many potters. They are normally manufactured by adding around 10% burnt or raw umber to an existing buff-burning cone 6 stoneware. Umbers are powerful colorants, they have high iron and also contain manganese (the latter being the primary source of the color). But these clays can be troublesome. First, good kiln venting is needed to avoid breathing the dangerous manganese metal vapors. Micro-bubble clouding/gloss-loss in the glazes and blistering/bloating of the bodies are common. But this mug fired perfectly. Why? The umber was added to a cone 10 stoneware instead (and it has fluxed the body to mature at cone 6). The mug has been white engobed on the inside and partway down the outside during leather hard stage. After bisque it was clear glazed on the inside giving a flawless surface (using G2926B) and dipped in GA6-A Alberta Slip base amber-clear. The GA6-A over the black clay produces a very deep, rich, almost black ultra-gloss surface.
Out Bound Links
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 applie...
There are decorative and functional reasons to glazing the outside and inside of mugs as a separate operation. First, liner glazes are far less likely to leach harmful things into food or drink. Liner...
Knowing about thixotropy enables you to mix non-gummed glazes that stay in suspension much better. But it is not only about staying suspended. While some glazes do not settle out they that have a slurry that behaves like a bucket of motor oil, silky smooth but they just drip and drip and drip. Thixo...
In Bound Links
Many potters do not think about leaching, but times are changing. What is the chemistry of stability? There are simple ways to check for leaching, and fix crazing.
Glazed ware can be a safety hazard to end users because it may leach metals into food and drink, it could harbor bacteria and it could flake of in knife-edged pieces.
The materials you use present two hazards you need to think about: Are they poisoning you while working with them? Are they destabilizing your glazes so they dissolve into food and drink?
Max Richens outlines the various mechanisms by which acids and bases can dissolve glass and glazes. He provides some information on stabilizing glazes against attack.
An example of how we can use INSIGHT software to determine of a glaze is likely to leach
Understanding the advantages of disadvantages of stains vs. oxide colors is the key to choosing the best approach
It is better to understand and have control of one good base glaze than be at the mercy of dozens of imported recipes that do not work. There is a lot more to being a good glaze than fired appearance.
Having Your Glaze Tested for Metal Release
Ask yourself the right questions to figure out the real cause of a glaze crawling issue. Deal with the problem, not the symptoms.
Crystal clear industrial dinnerware glaze
2013-09-26 - This is an industrial tableware glaze recommended by tech support at Fusion Frits. It not only fires hard and crystal clear but has outstanding suspen...
A base transparent glaze recipe created by Tony Hansen for Plainsman Clays, it fires high gloss and ultra clear with low melt mobility.
2014-02-06 - A cone 6 transparent general purpose base recipe developed at Plainsman Clays by Tony Hansen (see link to go there below, it contains technical and mi...
A base fluid-melt glaze recipe developed by Tony Hansen. With colorant additions it forms reactive melts that variegate and run. It is more resistant to crazing than others.
2015-09-30 - This is a test to publish Insight-live data here. Many more recipes coming soon.
Reliable widely used base glaze for cone 10 porcelains and whitewares. The original recipe was developed from a glaze used for porcelain insulators.
2003-12-18 - This is is long-time cone 10 transparent base, it is used by many potters around the world. It was originally employed as a high temperature porcelain...
A cone 04-02 clear glaze developed from Zero3 which in turn was developed from Worthington Clear. This employs frit instead of Ulexite.- 2016-10-26
A cone 10R dolomite matte having a pleasant silky surface, it does not cutlery mark, stain or craze on common bodies
2003-12-18 - A standard Plainsman Clays cone 10R dolomite matte glaze used for many years. Colorants and opacifiers can be added to create a wide range of beautifu...
By Tony Hansen