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G2934Y - G2934 (lower-LOI)

Modified: 2020-06-12 22:29:10

The same chemistry as the widely used G2934 but the MgO is sourced from a frit and talc instead of dolomite. It has a finer surface, less cutlery marking and staining.

Material Amount
Ferro Frit 324910.00
Nepheline Syenite10.50
Calcined Kaolin10.50


This is an "MgO matte". High MgO content is a common matting mechanism at cone 10 but also works at cone 6. This recipe has the same chemistry as G2934, however it sources the MgO from a frit and talc rather than from dolomite. The frit has a predictable chemistry, produces a much lower LOI (3.8 vs. 13.6), a smoother fired surface and better melt fluidity to host stains (stains look fantastic in this base).

Glazes that have MgO levels this high do tend to have crawling issues inside extreme concave contours (e.g. handle joins). Make sure wet glaze slurry wets all inside angles and is not applied too thick. And make sure the glaze dries quickly (glaze inside and outsides separately if needed to avoid water logging the bisque).

WARNING: Like the original, the degree-of-matteness is very dependent on cooling rate. Fast cooling (e.g. free-fall in a lightly-loaded or smaller kiln) produces a silky matte surface. Slower cooling (e.g. a heavily loaded kiln) produces a matter and drier surface (subject to cutlery marking). Test in your circumstances to decide whether to adjust your firing to accommodate this or blend in some glossy G2926B produce the silky surface.

Fusion Frit F-69 can be used instead of Ferro Frit 3249 (actually, it is better). This recipe employs a mix of calcined and raw kaolin to keep the drying shrinkage down (to avoid cracking on drying). If you use pure kaolin it will likely crack during drying. If you do not have calcined kaolin just make your own (bisque fire a container of powder).

This glaze has a very low thermal expansion and will not craze an any common clay body. It accepts stains exceptionally well.

We recommend doing cutlery marking tests on your ware. If they mark, blend in a little transparent glossy glaze (like G2926B). This will compromise the matteness a little but will reduce the marking.

Again, although matte in appearance, this glaze has a high melt fluidity. That means that brush-on colors could bleed.

Because this contains both calcined and raw kaolins you can adjust their ratios to control the compromise between dry hardness and drying shrinkage. If better gelling response to Epsom salts (or other flocculant) is needed trade one or two percent raw kaolin for bentonite (to increase the particle surface area).

1100 water, 1000 powder to get creamy mix.
Regular 2934 (with dolomite) took more water, about 1300. Nicer surface than G2934, same matteness, better fluidity.

Mixing notes:
Joe: April 29/19 Mixed up a batch and added 10% Mason #6021 Red stain. Final totals were: G2934Y + 10% stain weighed 5045 grams.
Water weighed 5450 grams.
Let glaze sit for 3 days to saturate before adding final water to bring to a S.G. of 1.43 and viscosity was approx. 12 seconds with ford cup. At these quantities, the final ratios would be:
Glaze powder (including stain) 48%
Water 52%
This mixture yielded about 7.3 litres of mixed glaze. This glaze was screened 80 mesh and has no Epsom salts added.

Related Information

An incredible silky matte surface supports wild colors at cone 6 oxidation

This 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). The orange color is brighter on the mug on the left because the porcelain is whiter, Plainsman Polar Ice (the other one is #6 Tile Kaolin based, P300). If this was a glossy glaze the required percentage of stain would be higher. Other colors, like yellow, are equally vibrant. But not all, testing is needed.

G2934Y glaze with stains on P300 and Polar Ice

Mugs hand-built by Tony Hansen. This base glaze is an adjustment to the original G2934 matte. It employs a frit to source the MgO instead of dolomite. The result is a glaze that melts and flows very well, yet is matte. And it is a great host for a wide range of stains, they look better in this than in a glossy base. The only color that has not worked well so far is purple.

Red stain in matte and glossy bases. Which looks best?

The silky matte. 11% Mason 6021 encapsulated stain has been added to each. The one on the left is G2926B glossy base (it also has 2% zircopax as a fining agent for the micro-bubbles). But in the matte base on the right, G2934Y, no zircopax is necessary. It produces a stunning surface using this and other stains (including orange, yellow, black, purple), very pleasant to touch.

Zircopax makes the liner glaze much whiter on this porcelain

Although this is a whiteware body (Plainsman M370), under the transparent liner glaze (G2926B) the color appears ivory, off white. But with 8% added zircopax the quality of the color is transformed into a white that is even better than what a much more expensive New Zealand or Grolleg kaolin porcelain would exhibit with a transparent cover glaze. The outer glaze is G2934Y matte with yellow stain. The glazes were applied inside and outside by pouring and dipping (it would not be possible to apply the yellow evenly enough by brushing).

Better to mix your own cover glazes for production?

Yes. In this case the entire outside and inside of the mug need an evenly applied coat of glaze. In production, it would not make sense to attempt this by painting. For these reasons: Cost, quality, convenience. The right pail has 2 gallons of G2934Y base with 10% Cerdec yellow stain: $135. Cost of jars with the same amount: Almost $300! And you have to paint them on in three coats with drying in between. The one in the pail is a true dipping glaze (unlike dipping glazes sold by glaze manufacturers that dry slowly and drip-drip-drip just like brushing ones). This one dries immediately after dipping in a perfectly even layer (if mixed according to our instructions). And a bonus: This pail can be converted to brushing or base-layering versions using CMC gum.

Cobalt and iron overglazes bleeding into the matte glaze

This is the G2934Y matte base with overglaze decoration fired at cone 6. Although this matte has a high melt fluidity, overglaze decoration can be successful as long as it is not applied too thick and not overfired. But in this case the glaze is thickly applied. Once the critical thickness boundary is passed, the glaze's ability to hold overglaze in place quick degrades. The G2934 recipe has less melt fluidity and fires to the same surface, it would would be a better choice as the base glaze in this case (and could be applied thicker). However this Y variation would be a good choice as the medium if you want to make your own overglaze brushable colors.

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

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.

Matte glaze cutlery marks. Add 10% glossy glaze to it. No marking.

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.

G2934 vs. G2934Y cone 6 yellow mattes

They have the same chemistry, but Y, on the right, sources MgO from a frit rather than from dolomite. Y is has a more fluid melt so it is not as opaque against the black slip underneath (thus it looks better on translucent porcelains). However its surface is just slightly finer. The Y one appears darker on the porcelain tile but it is actually brighter. The Y version is more likely to fire to a functional matte because it employs a frit to source the MgO (instead of dolomite which is variable from region to region).

Stunning black silky matte glaze at cone 6

This contains 6% Mason 6600 black stain (Mason 6666 gives dark brown, don't use it). The base recipe, G2934, is an excellent balanced-chemistry host for a wide range of stains to produce equally stunning reds, yellows, oranges, etc. The fritted version of the recipe, G2934Y, provides an even better host. This glaze is affected by the clay it is on. The body on the right is highly vitreous, this has produced a finer texture that glistens in the light. The body on the left is a whiteware having 1% porosity (Plainsman M370). Firing schedule is also a factor, slower cooling will dull the color more. We use the PLC6DS firing schedule.

The matteness of this glaze depends on the cooling rate

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.

Orange matte glazed porcelain mug

Slab-built. G2934Y base glaze recipe with 8% Cerdec encapsulated orange stain added. Cone 6. By Tony Hansen.

5% titanium dioxide in G2934Y matte, G1214Z matte, G2926B glossy

The body is Plainsman M390. The firing schedule is Plainsman PLC6DS.

Melt fluidity differences are not obvious by just comparing glazed ware

The glaze on two porcelain mugs looks the same, but the melt flow is very different

These two Plainsman M370 mugs were fired at cone 6, the left one with G2934 matte glaze, the right one with G2934Y4 matte. They look and feel identical in the hand. The two glazes have the same chemistry. But they employ different materials to source that chemistry. The secret of of the matteness is high MgO (magnesia content). In the glaze on the left that MgO is sourced by dolomite, a lot of it. The glaze on the right sources it from a special frit, Ferro 3249. The impact of this difference is visible in the melt flows, the fritted one is obviously melting and flowing better. On other clays, especially stonewares, the G2934 can have a dry surface that cutlery marks. Thicker applications make it worse. But the Y version exhibits no such issues. Its mattness, durability, cleanability and hardness are so good that it is being used in floor tile.

Cooling rate drastically affects the appearance of this glaze

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.

The glorious colors of G2934Y glaze

These are made from Plainsman M370 casting and fired to cone 6 using the PLC6DS firing schedule. There is a caution with thin cast ware: If walls are not thick enough to provide the porosity to be able to absorb all the water from the glaze, then the slow-drying glaze could crack and produce crawling during firing. Thinner pieces (not shown) did crawl in this firing.

Crawling in G2934Y zircon white glaze

G2934Y is a fabulous base glaze but it is not without issues. It has significant clay content in the recipe and high levels of Al2O3 in the chemistry, these make it susceptible to crawling. While it is normally fine as is, when you add certain stains to color it (especially at significant percentages) or opacify it using zircon (this has 10%), it can become more susceptible to crawling. On this mug, the glaze layer thickens at the recess of the handle join, that produces crawling during firing. This happens, both because the glaze cracked here during drying and because the zircon stiffens the melt, making it less mobile. To solve this issue one could use a mix of tin oxide and zircon (tin does not stiffen the melt, try 4% tin, 6% zircon, for example). Or, calcine more of the kaolin in the recipe (to reduce drying shrinkage).

Same body, same glaze, same firing. Why did one crawl?

The body: M370. Glaze: G2934Y (with added green stain). Firing: Cone 6 drop-and-hold. Glazing method: dipping (using tongs). Thickness: The same. The difference: Wall thickness. The one on the right was cast thinner so the glaze took a lot longer to dry (the bisque lacked sufficient absorbency). Common pottery glazes contain clays which need to shrink somewhat during drying. The bond with the bisque, although fragile, is normally enough to prevent cracking during drying if: drying occurs quickly. That happens when the body has enough porosity to absorb all the water quickly. Otherwise, cracks appear and these become crawls during firing. A complicating factor is that stain and/or zircon additions make an already-crawl-susceptible glaze even worse. Solution: Heat bisque before dipping, glaze the inside and outside separately (with drying between) or increase calcined kaolin:raw kaolin ratio in the glaze.

Emulating a speckled reduction fired stoneware in oxidation

The body is Plainsman M340S. Cone 6. Left to right: G1214Z calcium matte base glaze with 6% titanium dioxide added. GR6-A Ravenscrag base with 10% zircopax (zircon). G2926B glossy transparent base with 10% zircon (this one produces the white "Kohler Toilet Bowl" appearance we are seeking to better). G2934Y silky magnesia matte base with 10% zircon.

G2934Y glaze on Standard #112 body at cone 6

Produces an appearance very similar to dolomite-matte-glazed ware fired in cone 10 reduction. The effect would be similar using speckled bodies made by other manufacturers. Pieces made by Tom Friedman.

Our base glazes plus opacifiers on a dark burning body at cone 6

Red burning clay tiles with opacified versions of G2926B, G2934, G1214Z

The body is Plainsman M390. These are commonly-used base glazes. The top one is an MgO matte, next down is a calcium matte. They react very differently to these additions. Notice also the difference when titanium dioxide is applied thickly. Tin oxide fires whiter than zircon (e.g. Zircopax). Each opacifier has issues. Tin is expensive. Titanium is difficult to mix into the slurry (screening required), not as white or opaque, variations in thickness produce more difference in results and it can turn blue. Zircon is more likely to cutlery mark, twice as much is required and it amplifies the color of any iron present.


G2934Y variations for fired hardness, COE adjustment, etc
Recipes 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.

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