Digitalfire Ceramic Glossary



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Matte Glaze


A glaze that is not glossy. Of course, unmelted glazes will not be glossy, but to be a true matte a glaze must be melted and still not glossy. To be a functional matte it must also resist cultery marking, clean well and not leach into food and drink. Thus it is not easy to make a good matte glaze. It is common to see poor quality matte surfaces on name-brand table ware sold in major stores.

The vast majority of random material mixes that melt well want to be glossy. Matteness can be a product of the physical or mineral form of a material used, the chemistry and selection of materials to source that chemistry and often the firing schedule. While some types of mattes are stable, with others it can be difficult to maintain the same fired texture through material and firing variations. The best mattes are those whose mechanism is understood and have an adjuster (a firing change or a material whose percentage can be raised or lowered to fine tune the degree or character of the matteness).

The visual character of mattes, even those within the same mechanism, varies widely and is often difficult to characterize. Matteness is often part of a larger visual character that involves color and variegation.

Mechanisms that produce matte glazes produce surfaces that scatter light:
-Micro crystalline surfaces. High CaO glazes, for example, form minute calcium silicate crystals when cooling (at normal cooling rates). Wollastonite especially can do this, but also other sources of CaO. Another oxide that crystallizes well if oversupplied is ZnO, the size of the crystal being determined by the rate of cooling and level of ZnO.
-Micro-wavy or rippled (non flat) surfaces can be produced multiple ways. High Al2O3 (if supplied in a form that can decompose to enable Al2O3 to enter the melt), for example, stiffens the melt preventing level-out during cooling. Glaze melts that contain multiple melt phases solidify in a non-homogeneous way to produce a glass that both scatters light from within and from its surface.
-A special case of micro-rippled surfaces is MgO. It is a very effective matting agent at both high and middle temperatures. Talc and dolomite source the MgO to create this effect (although can differ in appearance). In higher temperatures the MgO creates multiple phases in the melt that have different fluidity and refractive indexes. These are sometimes called 'silky mattes' and are pleasant to the touch. Amazingly MgO also produces this effect at middle temperatures even though it is not an active melter there. Levels of 0.3 or higher will stiffen the melt without detrimentally affecting glass development and produce very pleasant matte surfaces. This works in boron fluxed glazes that have high or low Al2O3 and low or medium SiO2 levels, producing surfaces that do not cutlery mark and glazes that do not craze (because of the low expansion of MgO).
-Crowbar method! Materials whose individual particles are so refractory that they simply do not dissolve in the melt, if added judiciously to the right base, can produce a workable matte. Magnesium carbonate is an example. Even calcium carbonate, if supplied in raw form, does not melt at lower temperatures and can thus matte a glaze. But the best example is calcined alumina, if used in sufficiently fine particle size, can matte a glaze even with a small addition. However, alumina hydrate, by contrast requires a much greater addition. Why? It enters the chemistry of the melt and imparts a true alumina matte, the latter just increases the melting temperature because it is so refractory.
-Mechanisms that are not well understood. An example is barium mattes. Although they appear to be crystallized, some have found that no matter how fast they are cooled they still have the same degree of matteness. At the same time, fritted forms of the same amount of barium do not matte! In this system it appears the carbonate form supplies the BaO and seeds the crystals.

Employing combinations of these mechanisms is normally not practical because they can conflict. For example, a crystal matte is based on a highly fluid, well melting glaze, whereas an alumina matte is the opposite. However an exception to this is magnesia mattes, they can occur where alumina is high and silica is low (the alumina matte mechanism, although MgO can matte glazes that also have low MgO).

Functional matte glazes are more difficult to formulate (especially at middle and low temperatures) because they have a narrow window of chemistries or have recipes containing matting agents that are highly active (resulting in large changes in the degree matteness for small variations in the recipe or process). For crystal mattes, specific firing methods are also needed (e.g. slower cooling). Also, the degree to which mattes do not level out completely on cooling determines how easy-to-clean the surface of the glass will be.

Pictures

The reflection of light on a matte glaze

A refined-material cone 10R dolomite matte (left) vs. one made using Ravenscrag Slip

GR10-J Ravenscrag silky matte (right) and G2571A matte (left) on a buff stoneware at cone 10R. Surfaces feel identical, the slightly darker color is due to iron content in the Ravenscrag. The former was formulated to mimic the latter using as much Ravenscrag Slip as possible yet still maintain the same chemistry.

Look at recipes before wasting time and money on them.

Are they serious? This is a cone 6 melt flow comparison between a matte recipe, found online at a respected website, and a well-fluxed glossy glaze we use often. Yes, it is matte. But why? Because it is not melted! Matte glazes used on functional surfaces need to melt well, they should flow like a glossy glaze. How does that happen? This recipe has 40% nepheline syenite. Plus lots of dolomite and calcium carbonate. These are powerful fluxes, but at cone 10, not cone 6! To melt a cone 6 glaze boron, zinc or lithia are needed. Boron is by far the most common and best general purpose melter for potters (it comes in frits and gerstley borate, colemanite or ulexite; industry uses more boron, zinc and lithia frits). The lesson: Look at recipes before trying them.

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).

How to matte Ravenscrag Slip at cone 10 by adding talc

2,5,10,15% talc added to Ravenscrag Slip on a buff stoneware fired at cone 10R. Matting begins at 10%. By Kat Valenzuela.

2, 5, 10 and 15% calcined alumina added to Ravenscrag Slip

The Ravenscag:Alumina mix was applied to a buff stoneware fired at cone 10R (by Kat Valenzuela). Matting begins at only 5% producing a very dry surface by 15%. The matte is simply a product of the refractory nature of the alumina as a material, it does not disassociate in the melt to yield its Al2O3 as an oxide (as would a feldspar, frit or clay). The same test using alumina hydrate demonstrates that it disassociates better (although not completely).

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.

2, 5, 10 and 15% alumina hydrate added to Ravenscrag Slip

Pure Ravenscrag Slip is glaze-like by itself (thus tolerating the alumina addition while still melting as a glaze). It was applied on a buff stoneware which was then fired at cone 10R (by Kat Valenzuela). This same test was done using equal additions of calcined alumina. The results demonstrated that the hydrated version much more readily decomposes to yield its Al2O3, as an oxide, to the glaze melt. By 15% it is matting and producing a silky surface. However crazing also starts at 10%. The more Al2O3 added the lower the glaze expansion should be, so why is this happening? It appears that the disassociation is not complete, some of the raw material remains to impose its high expansion.

How to turn a dolomite matte white glaze into a bamboo matte

Make cone 10R bamboo colors using the GR10-J Ravenscrag silky matte base recipe (right) and adding 1% iron (left), (0.5% centre). These samples are porcelain. This iron addition also works using the G2571A matte base recipe.

Compare two glazes having different mechanisms for their matteness

These are two cone 6 matte glazes (shown side by side in an account at Insight-live). G1214Z is high calcium and a high silica:alumina ratio (you can find more about it by googling 1214Z). It crystallizes during cooling to make the matte effect and the degree of matteness is adjustable by trimming the silica content (but notice how much it runs). The G2928C has high MgO and it produces the classic silky matte by micro-wrinkling the surface, its matteness is adjustable by trimming the calcined kaolin. CaO is a standard oxide that is in almost all glazes, 0.4 is not high for it. But you would never normally see more than 0.3 of MgO in a cone 6 glaze (if you do it will likely be unstable). The G2928C also has 5% tin, if that was not there it would be darker than the other one because Ravenscrag Slip has a little iron. This was made by recalculating the Moore's Matte recipe to use as much Ravenscrag Slip as possible yet keep the overall chemistry the same. This glaze actually has texture like a dolomite matte at cone 10R, it is great. And it has wonderful application properties. And it does not craze, on Plainsman M370 (it even survived and 300F to ice water plunge without cracking). This looks like it could be a great liner glaze.

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).

A matte that is matte because it is not melting completely

Left: This specimen of VC71 cone 6 matte glaze was felt-marked and cleaned with acetone. A closeup of the ink specks reveals they are held in micro-bubbles breaking at the surface. This specimen has also been thermally stressed in a 300F/icewater test (causing the crazing pattern, which curiously, only shows up on part of the surface). Right: An adjustment to VC71 that adds more boron and Al2O3/SiO2 (while preserving the Si:Al ratio). It is much glossier, confirming that, even though the VC71 matte surface feels functional to the touch, it is a product of improper melting.

A good matte glaze. A bad matte glaze.

A melt fluidity comparison between two cone 6 matte glazes. G2934 is an MgO saturated boron fluxed glaze that melts to the right degree, forms a good glass, has a low thermal expansion, resists leaching and does not cutlery mark. G2000 is a much-trafficked cone 6 recipe, it is fluxed by zinc to produce a surface mesh of micro-crystals that not only mattes but also opacifies the glaze. But it forms a poor glass, runs too much, cutlery marks badly, stains easily, crazes and is likely not food safe! The G2934 recipe is google-searchable and a good demonstration of how the high-MgO matte mechanism (from talc) creates a silky surface at cone 6 oxidation the same as it does at cone 10 reduction (from dolomite). However it does need a tin or zircon addition to be white.

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, 10, 15 and 20% additions of the gloss recipe.

A true matte is still matte when you over fire it

The top glaze is VC71, a popular matte cone 6 glaze used by potters. Bottom is G2934 matte, a public domain recipe produced by Plainsman Clays. The latter is a high-MgO matte, it melts well and does not cutlery mark or stain easily. As evidence that it is a true matte, notice that it is still matte when fired to cone 7 or 8. VC71, while having a similar pleasant silky matte surface at cone 6, converts to a glossy if fired higher. This suggests that the cone 6 matteness is due to incomplete melting. For the same reason, it is whiter in color (as soon as it begins to melt and have depth the color darkens).

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By Tony Hansen




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