|Monthly Tech-Tip |
Random material mixes that melt well overwhelmingly want to be glossy, creating a matte glaze that is also functional is not an easy task.
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. It is even more common on hobby-made or potter-made ware.
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. For example, adjusting the cooling speed of the firing is almost certain to affect the degree of matteness. Or, adjusting the percentage of a material whose chemistry is pivotal to the matting mechanism. Or switching one frit for another. Or simply blending proportions of glossy and matte slurries to target a specific degree of 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. An advantage of matte surfaces is that they do not show fingerprints with bright or dark colors.
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). MgO has a high surface tension. In higher temperatures the MgO creates multiple phases in the melt that have different fluidity and refractive indexes. These are some times called 'silky mattes' and are pleasant to the touch. Amazingly MgO can produce this effect at middle temperatures even though it is not an active melter there (B2O3 assumes that role). Levels of about 0.35 MgO or higher can produce very pleasant matte 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 can be 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) and must be kept consistent. 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. Notwithstanding all of this, Al2O3 contributes to the hardness of fired glazes. Most matte glazes, by definition, have high Al2O3. Thus, if a specific matte melts well and does not cutlery mark, it could be among the most functional glazes available!
Companies generally configure their process to make the glaze as matte as possible while still having good technical properties (however some tolerate some cutlery marking). A semi-matte is a relaxing of the stringent requirements of the matte effect, a movement toward an easier-to-manufacture, more functional product. The semi-matte space is quite volatile, small chemistry changes, or even more important, firing changes, can produce large shifts in gloss of the fired glaze. Companies can measure the amount of semi-matteness by measuring the amount of reflected light from a glaze surface or comparison of surface micrographs.
This is G2934Y satin matte with Mason 6600 black stain. The piece on the left was fired using a slow-cool firing schedule (C6DHSC). The schedule for the one on the right turned the kiln off at 2100F (after a half hour drop-from-2200F-and-hold), then it free-fell. The slow cool gives the glaze on the left time to crystallize, thus it is now a stony matte (rather than a satin matte). It is interesting that to this mix of the glaze I added 20% glossy clear, yet it still matted on the slow cool.
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/ice-water IWCT 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.
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 GLFL test to compare melt-flow 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). Other melters used less often are zinc and lithium (or the fritted forms). The lesson: Look at recipes before trying them, we use the term limit recipe to describe the skill of being able to eye-ball a recipe and quickly assess if it is ridiculous or not.
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).
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%. This "psuedo matte" surface 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 somewhat better (although not completely).
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 suggest that the hydrated version is decomposing to yield some of 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, raw material remains to impose its high expansion.
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).
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).
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. 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 a 300F-to-ice water IWCT test). This looks like it could be a great liner glaze.
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.
On the right is the G2934Y matte base recipe with only 8% Cerdec Orange encapsulated stain. G2934Y employs a frit-source for the MgO (as opposed to G2934 which sources the MgO from dolomite). If this was a glossy glaze the required percentage of stain would be higher. Other colors (like yellow, red, blue, black), are equally vibrant. But not all (e.g. purple), testing is needed. The porcelain is Plainsman Polar Ice.
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.
These 10 gram balls were fired and melted down onto a tile. The one the left is the original G2934 Plainsman Cone 6 MgO matte with 6% Mason 6600 black stain. On the right the adjustment has a 20% glossy glaze addition to make it a little less matte. Notice the increased flow (the ball has flattened more) with the addition of the glossy. In addition, while the percentage of stain in the one on the right is actually less (because it was diluted), the color appears darker! Tuning the degree of matteness when making color additions to a base is almost always necessary to achieve a glaze that does not cutlery mark.
This is the G2934Y matte cone 6 recipe with a red stain (Mason 6021). The one on the left was fired using the C6DHSC slow-cool schedule. The one on the right was fired using the drop-and-soak PLC6DS schedule. The only difference in the two schedules is what happens after 2100F on the way down (the slow-cool drops at 150F/hr and the other free-falls). For this glaze, the fast cool is much better, producing a silky pleasant surface rather than a dry matte.
This is a calcium matte base (as opposed to the magnesia matte G2934). The clay is Plainsman M390. 5% Zircopax was added on the left (normally 10% or more is needed to get full opacity, the partially opaque effect highlight contours well). 5% tin oxide was added to the one on the right (tin is a more effective, albeit expensive opacifier in oxidation). The PLC6DS firing schedule was used.
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 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.
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.
G2000 - LA Matte Cone 6 Matte White
A silky zinc-fluxed matte used historically across North America
G2928C - Ravenscrag Silky Matte for Cone 6
Plainsman Cone 6 Ravenscrag Slip based glaze. It can be found among others at http://ravenscrag.com.
GR10-C - Ravenscrag Cone 10R Silky Talc Matte
Just Ravenscrag Slip plus 10% talc produces a visually variegated surface that feels silky and looks stunning!
G2571A - Cone 10 Silky Dolomite Matte Base Glaze
A cone 10R dolomite matte having a pleasant silky surface, it does not cutlery mark, stain or craze on common bodies
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.
G1214Z - Cone 6 Silky Matte
This glaze was born as a demonstration of how to use chemistry to convert a glossy cone 6 glaze into a matte.
How I Formulated a Cone 6 Silky Matte Glaze Using Insight-Live
I will show you how found a recipe on Facebook, assessed it, substituted my own materials, tested it, adjusted it. Now it is like a cone 10 dolomite matte.
Desktop Insight 2 - Creating a Matte Glaze
Learn to convert a glossy glaze into a matte by comparing its chemistry with a target matte formula. Alter the chemistry in such a way that the thermal expansion does not rise and it maintains good physical application and suspension properties.
In ceramics, reactive glazes have variegated surfaces that are a product of more melt fluidity and the presence of opacifiers, crystallizers and phase changers.
Ceramic glazes form crystals on cooling if the chemistry is right and the rate of cool is slow enough to permit molecular movement to the preferred orientation.
Phase separation is a phenomenon that occurs in transparent ceramic glazes. Discontinuities in the internal glass matrix affect clarity and color.
Ceramic glazes that mark from cutlery are either not properly melted (lack flux), melted too much (lacking SiO2 and Al2O3), or have a micro-abrasive surface that abrades metal from cutlery.
A formula ratio used to evaluate and predict firing properties in ceramic glazes.
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).
Refers to a group of ceramic fluxing oxides that contribute similar properties to fired glazes. They contrast with the alkalis which are stronger fluxes.
|Materials||Light Magnesium Carbonate|
Matte Glaze Recipes
Much less common that glossy glazes, normally have stricter firing requirements.
G1947U/G2571A Cone 10/10R Base Matte/Glossy Glazes
These starting recipes use no frits and work in oxidation/reduction and are inexpensive to make. They can be used as bases for the whole range of typical cone 10 pottery glazes (celadon, tenmoku, oatmeal, white matte, brown crystal).
A Textbook Cone 6 Matte Glaze With Problems
Glazes must be completely melted to be functional, hard and strong. Many are not. This compares two glazes to make the difference clear.