|Monthly Tech-Tip |
This is a frit based boron base glaze that is easily adjustable in thermal expansion, a good base for color and a starting point to go on to more specialized glazes.
Jan 2017: We recommend that you also read the documentation for G1916Q (link below). It employs different frits to accomplish the same objective and has a wider range of adjustment while remaining transparent.
The purpose of this page is not just to publish another recipe and throw you to the glaze dragon. For many the 'glaze recipe culture' and addiction to undocumented 'naked formulas' has meant countless 'blind alleys', years of wasted efforts, and gradual abdication of control to recipes that overstay their welcome and teach nothing. It has fostered a generation of ceramists with numbed consciences regarding their accountability for glazed ware they give or sell to others. The purpose here is to give you a 'starting point' so that you can exercise a degree of control over a base glaze to vary its color, surface, expansion, variegation, melting temperature, etc. (this site deals extensively with raising your expectations and understanding with regard to your glazes). The starting glaze recipe about which this page speaks is linked below. But instead of just going straight there, please read this background first.
One of the appeals of low fire and Majolica ware is that there is only one glaze to worry about. Generally artists employ a good transparent that they can add 15-20% zircon opacifier to to make a white. However, more than any other pottery process, technicians must be sure that their clay and glaze are compatible (have similar thermal expansions to minimize crazing and shivering). Even if the glaze does not visibly craze or shiver the already comparatively low strength of ware can be severely compromised.
Crazing and shivering are far more likely at lower temperatures because the less developed interface between clay and glaze is less able to absorb a bad fit. This is actually a blessing in disguise because it forces you to compensate. Crazing is more likely on a body made of clay materials only, shivering is possible on bodies containing talc.
The key to a good-fitting Majolica glaze is one word: Adjustability. Forget about mixing a whole bunch of recipes from others in the hopes that one will accidentally work. A methodical approach is far faster and more likely to produce a good product.
By 'adjustability' we mean you can alter the recipe to increase or decrease thermal expansion while not disturbing the glaze's visual properties or melting behavior. This gives you the ability to respond to any clay body. The best approach is to use a base recipe of 15 parts kaolin (EPK is best), 5 silica, and 80 of two or three balanced low melting borax frits having different expansions and 15% added zircon opacifier to make it white. Two frit examples are Ferro 3124 (expansion 7.7) and Ferro 3195 (expansion 7.1). Moderate shivering can be handled by simply increasing the amount of 3124 at the expense of 3195, crazing by increasing the amount of 3195 (frit 3195 also has a softer gloss). In the case of severe shivering, you can introduce a high expansion frit (such as Ferro 3134) or eliminate the silica, and for severe crazing increase the silica. Another thing worth mentioning is boron blue (see link to item in glossary). If you have this problem, increase the amount Al2O3 and reduce the CaO (frit 3195 is especially good for this).
One of the most difficult aspects of the Majolica process is firing ware to an even defect-free glaze surface. While an extended firing schedule is one aspect of this, it is also important to choose a proper clay body. Classical ware was made from red-burning clay. It might seem odd to struggle to evenly cover a red terra cotta surface with a white glaze rather than just use a white burning clay. However at low temperatures red burning bodies are much stronger simply because they melt at lower temperatures.
Get a terra cotta clay that fires to a nice red at cone 03. Try firing it to cone 02, 1, 3, and 5 to verify that it is a true terra cotta. It should be near melting by cone 5 and transitioning to dark brown around cone 1. Also, make sure the material has a fine particle size at least 100 mesh and finer (if there are larger particles in the body pinholing can occur because the larger particles both produce gases of decomposition and collect them in surrounding voids). If you have an electronic kiln controller you can extend firing time and tolerate coarser clays better if necessary.
'Dependability' is another big factor in majolica glazes. While frits are more expensive they are much more consistent and produce better slurries than Gerstley Borate mixtures. In addition Gerstley Borate recipes usually have no clear adjustment strategy, and even if there is other properties of the glaze are often detrimentally affected. Since the clay-glaze interface in low fire ware is not nearly as good as stoneware, the glaze bond will fail much more readily in functional use. Glazes tending toward shivering can shed sharp-edged glaze flakes, a catastrophic event if ware is being used functionally. Glazes tending toward crazing will severely compromise ware strength. The very best thing you can do is make sure your glaze fits as well as possible. If possible, get the clay/glaze combination strength-tested to make sure the glaze fits, even if there is no visible problem.
Make glaze text pieces and apply the glaze quite thick and fire to cone 06 and 04 and note the glaze's gloss and tendency to run. If it is running too much for cone 04, add a little kaolin (5%) to raise the melting temperature (if the glaze is also crazing add silica instead).
Perform a boil-ice test by immersing a fired glazed tile in ice water for five minutes, then boiling water for five minutes. Repeat this three or four times and check for crazing or shivering on each. If the glaze shivers keep increasing the higher expansion frit and redoing the test until it disappears. If you end up with all higher expansion frit, then remove the silica from the recipe and retest. If it still shivers begin introducing the super-high expansion frit (e.g. Ferro 3134). If the glaze crazes move in the opposite direction until all the frit is the lower expansion one. If it still crazes begin adding silica in 5% increments until the glaze does not melt properly. If it still crazes, fire higher and add more silica.
Low fire red clays often contain significant soluble salts that can deposit on the surface during drying, leaving an unsightly whitish scum and affecting the overlying glaze. Make sure your supplier is putting enough barium carbonate in the mix to precipitate these salts. Don't worry about barium leaching, it is under the glaze, it transforms to nontoxic barium sulfate during precipitation, and it participates in glass-building during the firing.
If necessary increase or decrease the amount of zircon opacifier if the glaze is either too white or not white enough.
Glaze must be applied evenly and much thicker than for porcelain or stoneware. Getting a thick even coverage of glaze is critical for the Majolica process. Any variation of glaze thickness or thinning at the edges will expose the underlying red body color or impose a darker glaze color (in stoneware this is not nearly as big of a concern). You must do whatever it takes to achieve an even thickness whether it be mixing 10 gallons so you can dip, spraying it, waxing and glazing in two stages, heating the ware before glazing, gelling the glaze with a few drops of an acid like vinegar, sanding away the drips, etc. If you need to add gum to harden the glaze surface for handling, be sure that the gum is not countering gelling tendencies or slowing drying as this will make it much more difficult to get an even coverage.
To begin production, we recommend you get a 50 lb. bag of frit (about $100) and mix 5-10 gallons of white and transparent glaze. It is always better to dip the ware completely to get an even layer. If you don't have a pair of dipping tongs, be sure to get some.
The character of the glaze slurry is critical to achieving even coverage. Using at least 15% kaolin with good gelling properties is one method. We find that EPK works very well. Make the slurry viscous enough so that you can dip your finger and pull it out slowly without one drip falling off.
Dip ware into the glaze using dipping tongs. Pull it out carefully and the glaze should stay put and dry within seconds for thick ware and in a few minutes for thinner walled items.
If you possibly can get an electronic kiln controller. The success of low fire ware, especially Majolica depends heavily on being able to fire up slowly and hold the temperature. Consistency of ware time after time depends on being able to repeat the same firing curve.
Obtain a selection of glaze stain powders recommended for cone 04 (in a pinch, you can also use copper carbonate for turquoise, cobalt for blue, and chrome for greens). Get a supply of foam cups, a teaspoon, and bag of the glaze developed above (minus the opacifier) to mix with the stains (some people use Gerstley borate). We will call this the 'melter' because many stain powders will not melt on their own. For each stain, take a foam cup and make an approximate 50:50 mix of the stain with the transparent melter. Add water and stir with your brush until it is a tempera paint consistency. Paint quick test-strokes on white glazed tiles. Cross some strokes to see what double thicknesses look like. With a fine brush paint the identification number of each stain using the stain mix itself. Fire the tiles.
Examine each color to determine if it is adequately melted. If not, increase the melter portion and test that color again. If any of the stains go on too dark or pasty, reduce the stain portion in favor of melter and retest. If any colors are not correct, it is possible that the chemistry of the melter conflicts with stain color development requirements (i.e. chrome-tin stains require the absence of zinc and at least 10% calcia, thus the Ferro frit 3134 above will not develop the pink if used alone in the glaze). Check documentation of the stains and compare with the chemical formula of your glaze recipe to solve these problems.
Paint some of each stain on a piece of bisque ware and apply the transparent version of the glaze over top (it does not need to be thick). Fire to see if the color develops properly. If any colors are not correct, again it is possible that the chemistry of the frit conflicts with stain requirements.
Because melting is a big issue at low fire it is much more difficult to create a matte that is hard, smooth, and uncrazed. Generally however, it is possible to simply add whiting, talc, dolomite, magnesium carbonate (or a mix) to a glossy glaze. While these materials supply CaO and MgO fluxes at high temperatures, at low fire they are refractory and stiffen and opacify the melt and precipitate out if they do dissolve. Try up to 20% magnesium carbonate and then reduce it until you get a compromise between matteness and resistance to crawling. Likewise, additions of 20 parts of mixtures of dolomite and whiting, or talc and whiting, or even dolomite alone will work in some glazes. Also, it is helpful to have plenty of silica present to encourage the formation of silicate crystals with CaO and MgO. Generally you need to experiment to get the right amount. Matte glazes will not heal imperfections as well and they will crawl if thick so you need to be prepared to learn how to use a particular mix effectively.
G2805 Cone 04-06 Matte Frit 3134 44 Dolomite 8 Kaolin 12 Whiting 8 Silica 28
Making the low fire process work requires considerable determination and testing, do not give up too easily. It is true that ware is not as strong as that fired at higher temperature, but it is also true that there is no other ceramic process that can give the same rich color and maintain the detail of even the finest brush strokes as low fire Majolica ware.
|Oxides||B2O3 - Boric Oxide|
Majolica is white opaque glazed red earthenware clay having colored overglaze decoration. But if you know more about what it is technically you will have more control of your product.
Ask the right questions to analyse the real cause of glaze crazing. Do not just treat the symptoms, the real cause is thermal expansion mismatch with the body.
|Materials||Ferro Frit 3134|
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