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How to Liner-Glaze a Mug

Section: Glazes, Subsection: Food Safety

Description

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.

Article Text

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.

Step-by-Step

  1. Use a pitcher to fill the bisque mug with liner glaze. Pour it out in a circular motion into the glaze bucket and then dip the lip into the glaze so it goes just barely covers the rim (perfecting this step will save you time in the next step). Hold it there long enough to get the thickness you want. This technique of pouring requires practice, here are some key points:
  2. After the glaze has dried sufficiently, use a banding or potters wheel to wax up the inside (starting about an inch down) and just up around the top of the lip. Be careful not to get any wax on the bare unglazed bisque.
  3. Using a sharp knife in one hand and rotating the mug with the other, cut away the glaze (down to the bisque) up to almost the highest point on the lip. This can be done with one smooth one-around rotation using the sharp knife. Clean up any remaining glaze down around the outside of the rim using a fettling knife. You could attach mugs to the wheel using a Giffin grip and cut the glaze off precisely, but that is too time-consuming, learn to do it by hand instead. When done you should have the inside glazed and terminating at a perfect wax-covered edge at the lip. For some glazes it may be necessary to clean any remaining glaze off the outside up to the wax line using a sponge (e.g. where the inside glaze is dark and the outside one is light). To minimize the amount of dust generated, do this operation before the glaze is too dry.
  4. Let the mug dry a while and glaze the outside. Keep your hands damp so they will not stick to the wax (and pull off sections as you touch it). Push the lip of the mug into the glaze just enough to get the join-line and the rim covered and down around it a few millimeters (be careful to hold the mug level as you do this). Immediately turn it over, put your hand inside, clamp against the interior surface, and push it into the glaze right down to the rim (to overlap the lip dip just made). When you push it into the glaze start at an angle, to make sure no air bubbles get trapped under the foot, and then straighten it as you push it down further. Hold it down for the time necessary to get the thickness you want, then pull it out quickly. Do a couple of downward jerks after you pull it down, then hold it at an angle to allow it any remaining drips to fall away. It may be tricky at first to push the mug down far enough and avoid glaze spilling into the inside, but once you get the knack it is easy.
  5. Set the mug on the table and sponge away any drips of glaze left over the wax-covered liner on the inside top of the lip. Clean the foot and set it out to dry.

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?

A matte and a glossy liner glaze

A matte and a glossy liner glaze

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.

Outside tenmoku glaze meets inside transparent in a straight line at the rim

Outside tenmoku glaze meets inside transparent in a straight line at the rim

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.

Three cone 10R mugs that have the same liner glaze.

Three cone 10R mugs that have the same liner glaze.

The liner is G2571A dolomite matte.

This cone 10R dolomite matte liner is also the base for the brown and blue

This cone 10R dolomite matte liner is also the base for the brown and blue

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.

Bisque-glazing vs. green-glazing in medium temperature porcelain

Bisque-glazing vs. green-glazing in medium temperature porcelain

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).

A once-fire mug vs. a bisque-fired mug

A once-fire mug vs. a bisque-fired mug

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.

Which liner glazing looks better to you?

Which liner glazing looks better to you?

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.

Glaze cracking during drying? Wash it off and change the process or glaze.

Glaze cracking during drying? Wash it off and change the process or glaze.

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).

A fluid melt glaze bleeds much more into adjoining ones

A fluid melt glaze bleeds much more into adjoining ones

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.

G3840 Shino on Grolleg/New Zealand kaolin porcelain at cone 10R

G3840 Shino on Grolleg/New Zealand kaolin porcelain at cone 10R

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.

Laguna B-Mix Cone 10R mugs with Alberta and Ravenscrag glazes

Laguna B-Mix Cone 10R mugs with Alberta and Ravenscrag glazes

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).

This leaching mug needs a liner glaze. Seriously!

This leaching mug needs a liner glaze. Seriously!

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.

A Cone 6 white engobe works miracles on these dark and buff burning bodies

A Cone 6 white engobe works miracles on these dark and buff burning bodies

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.

Mug made from a cone 6 black-burning stoneware body

Mug made from a cone 6 black-burning stoneware body

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.

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By Tony Hansen




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