|Monthly Tech-Tip |
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
Dolomite matte glazes have traditionally been fired around cone 10 and have a pleasant-to-the-touch silky-feeling surface. It is not the dolomite that produces the matte effect, it is the MgO in the dolomite. But, the name has stuck because dolomite has been the most common source of MgO. Other materials can also source it (e.g. talc, a frit). Thus the term "magnesia matte" is actually more correct. The unique feel is a product of discontinuities in the glaze melt (phase changes) that exhibit at the surface as tiny waves and ripples. MgO levels in the unity formula are typically 0.35 or more and normally accompanied by high Al2O3.
As noted, the phenomenon is often thought to be specific to high-temperatures, perhaps associated with the sudden melting of the dolomite particles and their conversion from a melt-stiffener to an enabler (by introducing discontinuous phases of more fluidity). But apparently this is not so, by the addition of B2O3 the MgO produces silky mattes of similar character in the cone 6 range. And it works especially well when some of the MgO is sourced from a frit (the needed level likely cannot be achieved from only a frit since this over-supplies B2O3). MgO 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). This actually makes it possible to produce an MgO silky matte at cone 04 (the only difference needed from the cone 6 version is a big increase in the level of B2O3).
In our observations, at high temperatures a low silica:alumina ratio is necessary to provide a viscous enough melt and enough KNaO is needed to get a good glass (since MgO normally enters at the expense of CaO). At lower temperatures likewise, the Al2O3 needs to be higher, SiO2 lower than in typical glazes.
At lower temperatures, MgO-as-a-refractory can stiffen an otherwise fluid glaze melt and produce a matte surface (although not normally silky). This happens when it is being sourced from raw dolomite or talc. This type of glaze falls outside of this discussion.
At higher temperatures, it can be tricky to produce a dolomite matte recipe that is also functional. Many are not sufficiently melted to produce a surface that resists cutlery marking and staining. Or, it is common to see high percentages of feldspar, producing crazing. And those to which colorants have been added can leach. It is best that MgO is not above about 0.35 molar in the formula, that adequate Al2O3 and SiO2 are present. This will require sufficient flux levels to melt it well. If your cone 10 dolomite matte is not working as you want, try comparing its chemistry with G2571A (cone 10).
The speed of kiln cooling can profoundly affect the surface. Slow-cooling (e.g. in heavily packed kilns) produces more matteness and fast-cooling more silkiness or gloss). Getting a specific surface becomes a question of whether to adapt the firing to the recipe, or the recipe to the firing.
By Tony Hansen
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.
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.
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.
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.
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 this with G2926B clear glossy (90:10 proportions) the marks are gone and the surface is only slightly changed.
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.
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.
Courtesy of Susan Clarke
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.
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.
Ravenscrag Slip based dolomite matte
Ravenscrag Slip + 10% talc = fantastic cone 10R silky matte glaze
Cone 10R dolomite matte effect at cone 6 oxidation
G2934 - Matte Glaze Base for Cone 6
A base MgO matte glaze recipe fires to a hard utilitarian surface and has very good working properties. Blend in the glossy if it is too matte.
G2928C - Ravenscrag Silky Matte for Cone 6
Plainsman Cone 6 Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.
Phase separation is a phenomenon that occurs in transparent ceramic glazes. Discontinuities in the internal glass matrix affect clarity and color.
A formula ratio used to evaluate and predict firing properties in ceramic glazes.
A high temperature reduction glaze made by adding a small amount of iron oxide to a magnesia matte base glaze
Random material mixes that melt well overwhelmingly want to be glossy, creating a matte glaze that is also functional is not an easy task.
|Oxides||MgO - Magnesium Oxide, Magnesia|
|By Tony Hansen|
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