All Glossary

Dolomite Matte

Dolomite matte glazes have the potential to be very silky and pleasant to the touch, while at the same time being hard, durable and non-crazed (if they are formulated correctly).

Key phrases linking here: dolomite matte, magnesia matte, mgo matte - Learn more

Details

Dolomite matte glazes have traditionally been fired around cone 10 and have a pleasant-to-the-touch silky surface. It is not the dolomite that produces the matte effect, it is the MgO in the dolomite, thus the term "magnesia matte" is actually more correct. But, the name has stuck because dolomite has been the most common source of MgO (talc and magnesium carbonate are others).

The characteristic soft satin surface can be explained in simple physical terms: A micro-wrinkle surface forms from a high viscosity, elastic melt that is too viscous and elastic to fully level on freezing. But there are additional mechanisms at play. A microstructure also develops while the melt is stiffening. That process needs time in the glass transition zone (~900–700 °C). Slow cooling stretches that zone out. Submicroscopic phase separation occurs as the glaze cools, the melt becomes less chemically comfortable holding MgO evenly dissolved in the silica-rich glass and it begins to separate into Mg-rich and an Si-rich glassy phases. These have different refractive indices and viscosities, light hits the boundaries and scatters. Fast cooling, by contrast, freezes the melt before this separation develops..

The minimum MgO level in the unity formula is typically 0.3 accompanied by high Al₂O₃ and a low Si:Al ratio (assuming slow cooling). For matte surfaces with faster cooling, MgO may need to be as high as 0.4 (and SiO₂ lower).

In our observations, at high temperatures a low silica:alumina ratio is necessary to provide a viscous enough melt. Enough KNaO is also needed to get a good glass (since MgO normally enters at the expense of CaO). The KNaO also helps to increase a thermal expansion that may otherwise be too low to fit bodies. Additions of B₂O₃ can flux the effect down to the cone 6 range (higher KNaO at the expense of CaO can lessen the boron requirements). This works especially well when some of the MgO is sourced from a frit. These frits are remarkable in that they supply a high proportion of the oxide yet melt at a far lower temperature than any MgO-containing natural material blend could do (and have zero-LOI compared to 45% for dolomite). With a significant increase in B₂O₃ it is even possible to produce the effect at cone 04.

At lower temperatures there is also recipe level matte mechanism with MgO. Talc, dolomite and magnesium carbonate are all refractory, their resistance to dissolving in the glaze melt can stiffen it and produce a matte surface (although not normally silky). This type of glaze falls outside of this discussion.

At stoneware temperatures, it can be tricky to produce a functional magnesia matte that resists cutlery marking, staining and leaching (one reason why my G2571A recipe is popular). The first challenge is that a viscous melt is a requirement so flux levels must be lower, this introduces the possibility of inadequate melting. Second, lower MgO and higher SiO₂ favours better functionality; the more firings can be control-cooled the more these are enabled. Getting a specific surface becomes a question of whether to adapt the firing to the recipe, or the recipe to the firing.

Related Information

Cone 10R dolomite matte glaze with 5% manganese dioxide

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By Tony Hansen

Dolomite bamboo matte glazed cone 10R mug

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Courtesy of Susan Clarke

A magnesia matte that breaks on contours

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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

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

Duplicating the G2571A dolomite matte using a Ravenscrag Slip based recipe

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GR10-J Ravenscrag silky matte (right) and G2571A matte (left) on a dark burning iron speckled stoneware at cone 10R. The 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.

The difference between dolomite and calcium carbonate in a glaze

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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.

Tuning the degree of gloss in a colored matte glaze

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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.

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

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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?

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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 that low percentages have with the higher strength that higher percentages enable. 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.

Matte base glaze cutlery marks.

Add 10% glossy glaze. No marking.

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This is G2934Y (a version of the G2934 cone 6 matte base recipe that supplies much of the MgO from a frit instead of dolomite). Like the original, it has a beautiful fine silky matte surface and feels like it would not cutlery mark. But, as you can see on the left, it does! The marks can be cleaned off easily. But still, this is not ideal. The degree of matteness that a glaze has is a product of its chemistry. But can we fix this without doing any chemistry? Yes. By blending in some G2926B clear glossy (90:10 proportions). The result: The marks are gone and the surface is only slightly less matte. This underscores the need to compromise the degree of matteness, on food surfaces, enough to avoid staining and cutlery marking.

Matte cone 6 glazes have identical chemistry but one melts more. Why?

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These are 10 gram GBMF test balls that we melted on porcelain tiles at cone 4 (top two) and cone 6 (bottom two). They compare the melt fluidity of G2934 (left) and G2934Y (right). The Y version sources its MgO from frit and talc (rather than dolomite). It is a much more fluid melt because the frit is yielding the oxides more readily. But Y has a key benefit: It has a much lower LOI, producing fewer entrained air bubbles and therefore fewer surface defects. And, even though it runs much more, it has the same matte surface! As long as it is applied at normal thickness, the extra melt fluidity does not cause any running. And it has another benefit: Less cutlery marking issues. It is actually a very durable and practical food surface glaze, having a low thermal expansion that fits almost any body. Although these appear glossy here, on ware they have the identical pleasant silky matte surface.

The power of calcined alumina to matte an MgO glaze

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This production batch of G2934 cone 6 MgO matte glaze is firing almost glossy (upper left). Matte glaze chemistries are generally sensitive - big variations in surface character can result from small changes in firing or material chemistry/physics. This recipe relies both on high MgO and lots of Al₂O₃ (from high dolomite and kaolin in the recipe). A change in the frit has crossed a tipping point. But there is an amazing fix: A small addition of super fine calcined alumina (400 mesh). How small? only 1%! A small change in the frit turned it glossy so this small change has fixed it. Another factor is that greater additions of alumina (shown here are 1.5, 2.0, 2.5, and 3.0%) progressively matte it more (the slow-cool C6DHSC firing is the reason for the opacity). If the alumina was not dissolving we would expect cutlery marking and surface staining. But neither is happening, even with additions of up to 8%, achieving stoney matteness.

Inbound Photo Links

Ravenscrag Slip based dolomite matte

Ravenscrag Slip + 10% talc = fantastic cone 10R silky matte glaze

Cone 10R dolomite matte effect at cone 6 oxidation

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