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G1214Z Cone 6 Matte Base Glaze

Section: Glazes, Subsection: Base Glazes

Description

This glaze was developed using the 1214W glossy as a starting point. This article overviews the types of matte glazes and rationalizes the method used to make this one.

Article Text

This page is a companion to the article on the cone 6 glossy G1214W base glaze recipe (which is in turn a derivative of the G1214M). Important information is available on the G1214W article and we suggest you study that page first before reading this one. The spirit of this article is not to produce the perfect matte, it is to talk about how Al2O3:SiO2 chemistry balance in a glaze recipe can be changed to adjust the degree of matteness of a glaze. Generally the best mattes are made using high MgO levels in an otherwise correctly melted base (see linked article on G2934, it is a better glaze).

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:

Unfortunately many matte surfaces out there are simply underfired glazes, they are not melted enough. Or, they are melted too much and surface micro-crystallization has gone too far. 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 often 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 often still be matte. However, keep in mind that the very fact that a glaze is matte means that it needs to push the limits. A stable glaze should be melted to a smooth surface and not crystallized. Matte glazes are normally neither. The mechanism of cone 6 matte that I will discuss here is often high CaO to encourage micro crystal development. If the CaO is within limits on most glazes they just will not 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 in 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 silica 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 will 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*
 SILICA           25.00  Al2O3  0.35
 F-4 FELDSPAR     15.00  B2O3   0.24*
                         SiO2   2.94
  Si:Al  8.69  Expan  6.62

See below for a link to this recipe.

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

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

See below for a link to this recipe.

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:

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 linked article).

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 high-alumina matte mechanism.

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. However, you can make an even better matte using the MgO mechanism and get a silky surface similar to the classic cone 10 dolomite matte (see the links for more info).

Possible Problems

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

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.

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

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

Pictures

A functional matte cone 6 glaze should melt as well as a glossy

True functional mattes have fluid melts, like glossy glazes. They need this in order to develop a hard, non-scratching durable glass. The mechanism of the matte on the right is high Al2O3 (G1214Z), it is actually melting more than the glossy glaze on the left (G1214W).

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By Tony Hansen




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