Modification Date: 2016-11-24 21:48:18
A base MgO matte glaze recipe created by Tony Hansen. It fires to a hard utilitarian surface and has very good working properties.
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This glaze is the product of a series of tests to determine the best levels of SiO2 and Al2O3 in a boron fluxed (but not boron saturated) clear using a high level of MgO to create the matte. In its current state it does not cutlery mark and has good (but not too much melt flow) and thus creates a good glass (in our circumstances). The development process for this glaze really showcases the value of looking at glazes as formulas of oxides rather than recipes of materials.
Although it has been believed that the cone 10R dolomite matte mechanism does not transfer well to cone 6, this glaze proves that wrong. That is why this recipe has such high dolomite (very unusual at cone 6). The calcined kaolin is needed to supply the Al2O3 but not increase the drying shrinkage. Notice also, that although a matte, this recipe has a high silica content, also very unusual. However, the matteness mechanism is somewhat fragile (it will be more matte for some people, more glossy for others; due to material, process and firing differences). In addition, certain colors will matte it more and others will gloss it more. For most people a simple addition of G2926B (M370 transparent ultra clear base recipe) will increase the gloss. Start by adding 10% and testing (since this and the gloss mix to about the same specific gravity, just measuring by volume of slurry is a fairly accurate way to judge the percent addition). Be prepared to adjust each color variation to fine tune its degree of gloss.
In our circumstances, this recipe has very good suspension and application properties. It dries hard, does not crack during drying, does not settle hard in the bucket and applies evenly and dries quickly. However, if it settles for you, just convert some of the calcined kaolin to raw kaolin in the recipe. However because the raw kaolin has a high weight loss on firing, more is needed to supply the same amount of SiO2 and Al2O3 to the fired glaze. For example, if you drop the calcined kaolin by 5, you need to increase the raw kaolin by 6.5 to maintain the same overall chemistry. That gives a kaolin amount of almost 25%, that should certainly suspend it. If not, you likely have electrolytes in your water that are deflocculating the glaze; use distilled water to make the glaze instead.
To prepare it for use, add the powder to water and mix until it flows well. Screen through 80 mesh (there are tiny agglomerates that will not break down without screening). Adjust the amount of water to get a consistency such that on extraction from a 1-2 second dip the glaze hangs on in an even layer without dripping and dries in less than 10 seconds. We have found that a 1.4 specific gravity is about right (gives a 13 second Ford Cup viscosity). The slurry needs to be slightly gelled to achieve this, so don't use too much water. Adjust your bisque temperature to get more porosity (if the glaze dries to slow) or less (if it dries too fast).
Although this is a matte glaze it flows well (it is well melted). If it pinholes for you the solution lies elsewhere. Bisque ware as high as possible. If you gel the slurry a little, or preheat the ware, you can bisque even higher. Try applying a thinner glaze layer and use whatever technique necessary to get an even and quality laydown. If still needed, consider using the double-soak firing schedule.
Plainsmanclays.com makes this recipe as a premixed powder. But they will also have a stock of the frit if you want to make your own.
G2934 is a popular matte for cone 6 (far left). It is not matte because it is not melting enough or is covered with micro-crystals, it is an MgO matte (a mechanism produces a more pleasant surface that cutlery marks and stains less). But what if it is too matte for you? This recipe requires accurate firings, did your kiln really go to cone 6? Proven by a firing cone? If it did, then we need plan B: Add some glossy to shine it up a bit. I fired these ten-gram balls of glaze to cone 6 on porcelain tiles, they melted down into nice buttons that display the surface well. Top row proceeding right: 10%, 20%, 30%, 40% G2926B added (100% far right). Bottom: G2916F in the same proportions. The effects are similar but the top one produces a more pebbly surface.
These are Mason stains added to the cone 6 G2934 silky MgO matte liner base glaze (with tin, zircopax and various stains added). The brightest colors (6600, 6350, 6300, 6021, 6404) were tested overnight in lemon juice without visible changes. It is known that MgO mattes are less prone to acid attack that CaO mattes. Caution is required with inclusion stains (like #6021), if they are rated to cone 8 they may already begin bubbling at cone 6 is some host glazes.
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.
Left: a cone 6 matte glaze (G2934 with no colorant). Middle: 5% Mason 6006 chrome-tin red stain added. Right: 5% Mason 6021 encapsulated red stain added. Why is there absolutely no color in the center glaze? This host recipe does not have the needed chemistry to develop the #6006 chrome-tin color (it lacks sufficient CaO and has alot of MgO). Yet this same matte glaze intensifies the #6021 encapsulated stain at only 5% (using 20% or more encapsulated stain is to develop the color is not unusual).
G2934 cone 6 matte (left) with 10% zircopax (center), 4% tin oxide (right). Although the cutlery marks clean off all of them, clearly the zircopax version has the worst problem and is the most difficult to clean. To make the best possible quality white it is wise to line blend in a glossy glaze to create a compromise between the most matteness possible yet a surface that does not mark or stain.
Left: G2934 cone 6 matte glaze with 3% Mason 6300 blue stain. Right: An additional 4% tin added. Notice how an opacified color does not have depth and therefore is lighter in color. Also it does not break to different shades at the edges of contours the way the transparent color does.
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% 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 is actually less (on the right), 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.
An example of how a spoon can cutlery mark a glaze. This is a popular middle temperature recipe used by potters. The mechanism of its matteness is a high percentage of zinc oxide that creates a well-melted glaze that fosters the growth of a mesh of surface micro-crystals. However these crystals create tiny angular protrusions that abrade metal, leaving a mark. Notice the other matte flow on the left (G2934), it not only has a better surface (more silky feel) but also melts much less (its mechanism is high MgO in a boron fluxed base).
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 10 gram balls of four different common cone 6 clear glazes. I fired them to 1800F (bisque temperature) to see how dense they would be. Why? To answer this question: If the gases of decomposing lignite have not been fully expelled from the clay body during bisque, then could a glaze densify enough to seal the surface from that temperature up? The answer appear to be yes. I measured the porosity of these (weighing, soaking, weighing again): G2934 - 21%. G2926B - 0%. G2916F - 8%. G1215U - 2%. The implications: Glaze pinholes in improperly bisqued ware.
These cone 6 porcelain mugs have glossy liner glazes and matte outers: VC71 (left) crazes, G2934 does not (it is highlighted using a felt marker and solvent). Crazing, while appropriate on non-functional ware, is unsanitary and severely weakens the ware (up to 300%). If your ware develops this your customers will bring it back for replacement. What will you do? The thermal expansion of VC71 is alot higher. It is a product of the chemistry (in this case, high sodium and low alumina). No change in firing will fix this, the body and glaze are not expansion compatible. Period. The fix: Change bodies and start all over. Use another glaze. Or, adjust this recipe to reduce its thermal expansion.
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.
Left: G2934 magnesia cone 6 matte (sold by Plainsman Clays). Right (G2934D): The same glaze, but with 0.4 molar of BaO (from Ferro Frit CC-257) substituted for the 0.4 MgO it had. The MgO is the mechanism of the matte effect. Barium also creates mattes, but only if the chemistry of the host glaze and the temperature are right. In addition, barium mattes are normally made using the raw carbonate form, not a frit. In fritted form, barium can be a powerful flux when well dissolved in the melt and boron is present. This glaze is actually remarkably transparent. However, if this was fired lower it could very well matte.
These mugs have just finished immersion into ice water from 300F. Twice. The LA Matt is crazing but the G2924 is still good. And its surface is more silky and more pleasant to the touch. It is whiter because of a 4% tin oxide addition. This is a glaze surface that would be excellent on most cone 6 porcelains. Remember, if you need to adjust the matteness, just add a little glossy to the batch.
We are looking at two pairs of samples, they demonstrate why knowing about glaze chemistry can be so important. Both pairs are the same glazes: G2934 cone 6 matte and G2916F cone 6 glossy. The left pair has 5% maroon stain added, the right pair 5% purple stain. The red and purple develop correctly in the glossy but not the matte. Why? The Mason Colorworks reference guide has the same precaution for both stains: the host glaze must be zincless and have 6.7-8.4% CaO (this is a little unclear, it is actually expressing a minimum, the more the CaO the better). The left-most samples of each pair here have 11% CaO, the right-most have 9%. So there is enough CaO. The problem is MgO (it is the mechanism of the matteness in the left two), it impedes the development of both colors. When you talk to tech support at any stain company, as I did with Mason on this, they need to know the chemistry of your glaze to help, not the recipe.
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By Tony Hansen