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A Cone 6 Matte Glaze

This page is a companion to our page on the cone 6 glossy G1214W glaze recipe (which is in turn a derivative of our G1214M recipe). Important information is available on the G1214W page and we suggest you study that page first before reading this one.

There is a tendency to think that good matte glazes are only possible at high temperatures and that those from all other temperature ranges are second best. This is certainly not true with reference to cone 6. Glazes can be matte for a variety of reasons and these reasons are largely independent of temperature. For example:

A fluid melt can form a fine crystalline 'skin' during cooling and these crystals scatter light like little prisms.
Glazes high in alumina tend to form a 'wavy' or 'rippled' micro-surfaces that scatter light.
Glazes with the proper mineralogy and particle size distribution can melt in a non-homogenous way producing rivulets of fluid glass and islands of stiffer less melted material. The resultant micro-surface is a non-homogeneous mix of different light reflectance properties.

Unfortunately many or most of the matte surfaces out there are simply underfired glazes, they are not melted enough. A true matte has a specific chemistry that does not happen easily by random material blending because the vast majority of materials are silica-dominant and thus by nature produce glossy glazes. In addition, crystal mattes that are discovered by chance invariably craze on the body. A true matte is one that is both matte and still functional, that is, it is hard, fits the clay body, does not cutlery mark, is resistant to leaching, etc. A true matte will usually fire higher and still be matte.

As implied, mattes by nature tend toward crazing and cutlery marking. Visit a fine dinnerware store and you will see very few matte glazes. If you do take out one of your keys and try to mark the surface and then rub it off and see what happens. It is a fact that even major players in the ceramic industry have, to some extent, depended on customer ignorance regarding matte surfaces. Worse yet, other companies and individual potters themselves are less than knowledgeable about how to formulate and fit a good matte glaze.

Matte glazes, especially at cone 6, tend to expose the folly of the prevailing mindset in university and industrial ceramics. People tend to pin their fortunes on the traffic in recipes. However dependence on this traffic spawns helplessness and delivers barium mattes whose safety is surrounded by controversy; crystalline mattes which fire inconsistently, craze and work unpredictably with colors; or improperly melted compounds that are not functional, durable or hard. Most important true high alumina mattes don't travel well because they are inherently volatile, that is, the degree of matteness varies disproportionately with small changes in formulation, firing, body and additions. In fact it is not uncommon for a glaze that fires matte in one kiln to
fire glossy another.

Thus matte glazes are an excellent candidate for an approach we advocate: Formulate or adapt a good base glaze, understand it, adjust it to achieve a variety of matte colors and surface effects. I'll say it another way: A matte glaze should have a documented way to tune the degree of matteness desired. Ideally it should have some flint in the recipe that can be cut for greater matteness or increased for greater silkiness and gloss. Typically this will be a silky matte just beyond the cutlery
marking stage.

To best 'understand' the recipe I recommend starting with a reliable transparent glossy glaze and adjusting it toward matteness. The process of working through this will give you a good understanding, you'll see the more difficult matte in terms of the glossy base that you already know. Let us do that now. Here is a glossy base glaze that I have been working with. It is thoroughly dealt with in its own article.

G1214W CLEAR LOW EXPANSION
 WOLLASTONITE     10.00  CaO    0.57*
 FRIT 3134        25.00  K2O    0.02*
 PIONEER KAOLIN   25.00  Na2O   0.17*
 FLINT            25.00  Al2O3  0.35
 F-4 FELDSPAR     15.00  B2O3   0.24*
                         SiO2   2.94
  Si:Al  8.69
  Expan  6.62

For a web page showing this recipe with Hyperlinked materials and oxides (i.e. if you need to find substitute materials in your area), please visit http://digitalfire.com/recipes. To research materials in general visit http://digitalfire.com/material, to research oxides visit http://digitalfire.com/oxide.

To make this glaze into a matte I did the following:

Eliminated the flint. Why? Alumina matte glazes have high alumina and low silica, thus they have a low silica:alumina ratio in the formula. This one change reduces the ratio from 9:1 to 5:1. While this ratio is a perfect target for matte effects, the change has introduced a problem that illustrates why good matte glazes don't often happen by accident: The thermal expansion INSIGHT calculates sky-rockets almost guaranteeing that it would craze.
Eliminated the feldspar. Why? Feldspar contributes lots of sodium oxide (Na2O) and potassium oxide (K2O) and these have by far the highest thermal expansions. This change means that other materials must be employed to restore fluxes, SiO2 and Al2O3 lost by cutting the feldspar.
Switched to an alumina-containing boron frit (the one in the starting glaze contains no alumina). I also increased the frit amount enough to take B2O3 up to 0.3, this makes up for the lost K2O and Na2O.
Adjusted the kaolin and silica to achieve a low ratio.

G1214Z
 WOLLASTONITE   27.00  CaO    0.88*
 FRIT 3124      36.00  K2O    0.01*
 EPK KAOLIN     35.00  Na2O   0.10*
 FLINT           5.00  Al2O3  0.47
             ========  B2O3   0.19
               105.00  SiO2   2.51
  Si:Al  5.78
  Expan  7.10

It is true this recipe has a higher calculated expansion than its glossy parent. However remember that calculated expansions are relative within the same oxide/material/firing system only, I have moved to a glaze with a drastically different balance of oxides. My tests indicate that an expansion of 7.1 for this 'system' is low enough not to craze on any of the 10 or more middle temperature clay bodies I use.

To the observer these are two completely separate glaze recipes. But to me they are not, I see the matte as an adjustment to the glossy over which I have great control. Notice some benefits of this recipe:

It has plenty of EPK, this makes the slurry a dream to use, it goes on silky without any drips.
It has a simple recipe.
It has a little flint which means I can reduce or increase it over time to fine tune the fired surface.
It has enough calcium that it works with chrome-tin colors.
It has a very low calculated thermal expansion.

Remember, since many colorants are fluxes they have the potential of turning a matte glaze glossy. Thus once you have tuned the base recipe to the amount of matteness you want you need to retune it for each variation (addition of color, opacifier, variegator). In each case we recommend you leach test it (see http://digitalfire.com/education/glaze/leaching.htm).

You may be wondering why I started with a glossy glaze and adjusted it toward matteness rather than improving an existing matte or starting from scratch. The reason is that I want to be able to rationalize the matte recipe in terms of the glossy and thus better understand the mechanisms that produce the matte surface quality.

Further Work

Can you find a way to reduce the CaO, maintain the thermal expansion, and still achieve a silky matte surface. Some users have demonstrated that it is possible to increase the alumina far higher (boron glazes can take a lot of alumina, especially if ball milled well) thus making room for more KNaO while still maintaining a low thermal expansion. It should also be possible to increase the boron, reduce the alumina and hold the glaze up with high MgO to get a surface with the classic cone 10 dolomite matte touch.

Possible Problems

Alumina matte glazes by nature have a few potential problems you must consider:

Since this recipe has a fairly high kaolin amount (to supply the alumina) it is possible it may shrink excessively and crack during drying. If this happens to you then either use a less plastic kaolin, use a calcined kaolin:raw kaolin mix to reduce drying shrinkage or adjust the formula to supply some of the Al2O3 from other materials so the kaolin amount can be cut. Also, if you are in the habit of putting bentonite in all of your glazes, don't put it in this one; bentonite is only needed in glazes lacking clay materials to suspend and harden them.
Alumina mattes have, by nature, a stiff melt with high surface tension. That means the glaze will not 'wet' non-vitreous bodies as well, it will not flow into tight recesses, and when it bubbles they will resist healing or even resist bursting to the point that a pyro-plastic glaze skin swells away from the body and then wrinkles back down during cooling. If your glaze wrinkles we suggest you minimize the amount of added zircon opacifier (these stiffen the glaze further) and use tin oxide instead, cut the Al2O3 a little, use a body with a vitreous surface to bond better with the glaze, and experiment to find a sympathetic firing curve.
If the glaze is not properly melted it will be somewhat absorbent and thus will hold stains and be difficult or impossible to clean. To test for this paint ink or a strong dye (i.e. methyline blue) onto the surface and attempt to clean it off.

Ball Milling

Matte glazes, because they do not flow as much when melted, can exhibit surface imperfections that are sometimes difficult to deal with. Any particulate contaminant that does not dissolve in the glaze melt is a potential source of blisters, pinholes, etc. Likewise bodies containing larger particles that produce gases during firing will disrupt the glaze surface. These problems can be erratic and very frustrating. Thus if you are a production potter or industrial user do not ignore the value of a ball mill. In addition, be sure to mill for an adequate period. Depending on the efficiency of your mill and your tests it may be that you need to mill for 10 hours or more (yes, 10 hours).

A ball mill breaks mineral particles down to very small sizes and thus the glaze as a whole melts much better and wets the surface of the glaze more effectively. You will find that a milled version of this glaze will have a more velvety surface and may be more glossy than its unmilled counterpart. Thus it will likely be possible to raise the alumina and silica even higher to get better hardness and lower thermal expansion.

If you would like to go to a large web site dedicated to this kind of philosophy then visit our home page.

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