Digitalfire Ceramic Glossary



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Dolomite Matte


Dolomite matte glazes are normally fired at cone 10 and often have a very pleasant-to-the-touch silky-feeling surface. This unique feel is a product of tiny discontinuities in the glaze melt (phase changes) that exhibit at the surface as tiny waves and ripples. This phenomenon is thought to be associated with the sudden melting of MgO-sourcing particles and their conversion from an element that stiffens the melt to one that makes it more fluid (by introducing discontinuous phases of more fluidity). While the host glaze can have CaO, it needs to be at the low end of its normal range and the MgO needs to be at the high end. In addition a low silica:alumina ratio is necessary to provide a viscous enough melt and enough KNaO is needed to get a good glass. Dolomite normally sources some or all of the MgO, thus the term "dolomite matte" ("magnesia matte" is actually more correct, since talc sources MgO to produce the same effect). Thus, theoretically, it is not possible to significantly reduce the firing temperature of a specific cone 10 dolomite matte (since the mechanism would be lost). Notwithstanding, MgO can produce the silky effect at lower temperatures anyway! At cone 6, for example, in a low CaO and SiO2 glaze (also having plenty of Al2O3 to stiffen the melt and enough B2O3 flux), MgO will micro-wrinkle the surface to produce a silky matte! Dolomite does not work well to source the MgO at cone 6, talc should be used (thus the term "Talc Matte"). Even at low temperatures, MgO, as a refractory, can stiffen an otherwise fluid glaze melt and produce a matte surface (although not normally silky).

If can be tricky to produce a dolomite matte that is also functional (does not craze, cutlery mark or leach). Many dolomite matte glazes are not sufficiently melted to produce a surface that cannot be marked. It is common to see high percentages of feldspar, these glazes often craze. Also, when the MgO is too high the surface can become too dry or the glaze may be volatile, firing OK in one firing but too dry in another. But when the proportions are right, these glazes can be quite stable and fire to the same appearance firing after firing.

If your dolomite matte is not working as you want, try comparing its chemistry with G2571A, referenced on this page.

Pictures

Dolomite bamboo matte glazed cone 10R mug

Courtesy of Susan Clarke

Cone 10R dolomite matte glaze with 5% manganese dioxide

By Tony Hansen

A magnesia matte that breaks on contours

GR10-G Silky magnesia matte cone 10R (Ravenscrag 100, Talc 10, Tin Oxide 4). This is a good example silky matte mechanism of high MgO. The Ravenscrag:Talc mix produces a good silky matte, the added tin appears to break the effect at the edges.

2, 5, 10, 15% dolomite added to Ravenscrag Slip at cone 10R

This is a buff stoneware clay. Crystal development toward a dolomite matte begins at 15%. By Kat Valenzuela.

Two great dolomite matte cone 10R recipes on iron stoneware

GR10-J Ravenscrag silky matte (right) and G2571A matte (left) on a dark burning iron speckled stoneware at cone 10R. Surfaces have identical feel (the chemistries are very close). The former fires a little darker color because of the iron contributed by the Ravenscrag Slip.

Ravenscrag dolomite matte

GR10-J Ravenscrag dolomite matte base glaze at cone 10R on Plainsman H443 iron speckled clay. This recipe was created by starting with the popular G2571 base recipe (googleable) and calculating a mix of materials having the maximum possible Ravenscrag Slip percentage. The resultant glaze has the same excellent surface properties (resistance to staining and cutlery marking) but has even better application and working properties. It is a little more tan in color because of the iron content of Ravenscrag Slip (see ravenscrag.com).

Tuning the degree of gloss in a colored matte glaze

Matte glazes have a fragile mechanism. That means the same recipe will be more matte for some people, more glossy for others (due to material, process and firing differences). In addition, certain colors will matte the base more and others will gloss it more. It is therefore critical for matte glaze recipes to have adjustability (a way to change the degree of gloss), both for circumstances and colors. This recipe is Plainsman G2934 base matte with 6% Mason 6600 black stain added. It has been formulated to be on the more matte side of the scale so that for most people a simple addition of G2926B (M370 transparent ultra clear base recipe) will increase the gloss. That means users need to be prepared to adjust each color of the matte to fine-tune its degree of gloss. Here you can see 5:95, 10:90, 15:85 and 20:80 blends of the matte:gloss recipe bases.

The difference between dolomite and calcium carbonate in a glaze

These glaze cones are fired at cone 6 and have the same recipe: 20 Frit 3134, 21 EP Kaolin, 27 calcium carbonate, 32 silica. The difference: The one on the left uses dolomite instead of calcium carbonate. Notice how the MgO from the dolomite completely mattes the surface whereas the CaO from the calcium carbonate produces a brilliant gloss.

A magnesia speckle matte at cone 6 oxidation is impossible, right? Wrong!

I am getting closer to reduction speckle in oxidation. I make my own speckle by mixing the body and a glossy glaze 50:50 and adding 10% black stain. Then I slurry it, dry it, fire it in a crucible I make from alumina, crush it by hand and screen it. I am using G2934 cone 6 magnesia matte as the glaze on this mug on the left. To it I added 0.5% minus 20 mesh speck. Right is a cone 10R dolomite matte mug. Next I am going to screen out the smallest specks, switch to a matte glaze when making the specks (they are too shiny here), switch to dark brown stain. Later we will see if the specks need to bleed a little more. I am now pretty well certain I am going to be able to duplicate very well the reduction look in my oxidation kiln. I will publish the exactly recipe and technique as soon as I have it.

Looking for a non-crazed non-cutlery marking cone 10R dolomite matte?

This is G2571A cone 10R dolomite matte on an ironware body made from native North Carolina clays. Few glazes have the pleasant silky feel that this has yet are still functional. The feldspar content in the body has been tuned to establish a compromise between the warmer color low percentages have with the higher strength that higher percentages impart. Careful porosity tests were done and recorded in an account at insight-live.com. The objective was to bring the body close to 3% absorption.

Out Bound Links

  • (Glossary) Phase Separation

    This phenomenon is responsible for some of the mos...

  • (Oxides) MgO - Magnesium Oxide, Magnesia
  • (Glossary) Matte Glaze

    A glaze that is not glossy. Of course, unmelted gl...

  • (Glossary) Semi-Matte Glaze

    It is difficult to draw a line between what is mat...

  • (Recipes) G2928C - Ravenscrag Silky Matte for Cone 6
    Plainsman Cone 6 Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.
    2014-02-20 - This works well on Plainsman M340, but especially ...

In Bound Links


By Tony Hansen




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