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G2934Y - G2934 (lower-LOI)

Modified: 2024-12-16 17:03:49

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.

Notes

This recipe was made as an alternative to blending G2934 matte and G2926B glossy to tune the degree of matteness (this being equal to about an 85:15 mix of those). Those two recipes do not require Frit 3249, they use the much more common 3134, 3124 and 3195. If you are getting started that approach would likely be better for you.

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 Ferro frit 3249 and talc rather than from dolomite. This specific frit is the secret to why this glaze works. If you do not have it and do not know how to do the chemistry to substitute another MgO-sourcing frit, then you must use the G2934 recipe.

The use of a frit to source MgO in this recipe 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, including this one, can have crawling issues with certain stains or opacifier additions, especially 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 on any common clay body. It accepts stains exceptionally well. It is low enough that on some clays it will be under too much glaze compression, this could lead to dunting. Use G2934Y2 if this is an issue.

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 a 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 the glaze sit for 3 days to saturate before adding final water to bring to an 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

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

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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.

Stunning black silky matte glaze at cone 6

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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.

G2934Y glaze on Standard #112 body at cone 6

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This silky matte glaze produces an appearance very similar to dolomite matte glazed ware fired in cone 10 reduction. The degree of matteness can be controlled by the cooling rate of the firing. Although this body is made by Standard Ceramics, the effect would be similar using speckled bodies made by other manufacturers also. These pieces made by Tom Friedman.

G2934 vs G2934Y melt fluidity

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The difference is quite amazing. The chemistry is the same. But the MgO is much more readily released from the frit in the Y version (vs the dolomite in G2934). Also, even though the melt is more fluid, it is still just as matte (a testament to the fact that the matteness in this recipe is not because it is improperly melted). Part of the reason for the extra fluidity could be the lower entrained micro-bubble population in the Y (these possibly impeding the flow of the dolomite version on the left).

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

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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.

Melt fluidity differences are not obvious by just comparing glazed ware

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These two Plainsman M370 test 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 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 fluidity tester, the fritted one is melting and flowing much 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.

Matte base glaze cutlery marks.

Add 10% glossy glaze. No marking.

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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 in some G2926B clear glossy (90:10 proportions). The result: The marks are gone and the surface is only slightly less matte. This underscores the need to compromise the degree of matteness, on food surfaces, enough to avoid staining and cutlery marking.

The matteness of this glaze depends on the cooling rate

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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.

Cooling rate drastically affects the appearance of this glaze

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This is the G2934Y satin matte glaze recipe with Mason 6600 black stain (6%). The piece on the left was fired using the C6DHSC firing schedule (drop-and-hold at 2100F then 150F/hr to 1400F). The one on the right was fired using the PLC6DS schedule (drop-and-hold at 2100F then free-fall from there). The slow cooling rate gives the glaze on the left time to crystallize, creating a stony matte (and altering the colour accordingly). My kilns are generally lightly loaded, so free-fall firings drop rapidly, producing the effect on the right. This phenomenon is a characteristic of high MgO glazes (ones having significant dolomite, talc, Ferro frit 3249). To vary, by recipe, the degree of matteness, we also make this glaze using a blend of G2934 base (which fires even more matte on slow-cool) and G2926B glossy (starting with and 80:20 matte:glossy mix). Of course, this type of glaze would not be practical in an industrial shuttle kiln, pieces would fire differently depending upon their placement on the cars.

G2934Y glaze with stains on P300 and Polar Ice

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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.

G2934 cone 6 DIY MgO matte glaze: Reliable, durable, adjustable, stainable

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These pieces were made from Plainsman Polar Ice and fired to cone 6 using variations on the PLC6DS and C6DHSC schedules. The dipping glaze is G2934Y, a recipe variant of G2934 having a finer micro-surface texture (it has the same chemistry but the MgO is sourced from a frit and talc instead of dolomite). These mugs display how well the recipe works with stains and the varying degrees of matteness we can achieve by varying the cooling rate and the percentage of glossy G2926B base blended in. As a magnesia matte, this glaze can have a surface that pleasant to the touch. It fires durable, can be quite matte without cutlery marking, and it has very good slurry and application properties (as a dipping glaze). It has a very low thermal expansion (won’t craze). It works really well with stains (except purples). It melts even better than the glossy!

G2934 vs. G2934Y cone 6 yellow mattes

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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).

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

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The body: M370. Glaze: G2934Y (with added green stain). Firing: Cone 6 drop-and-hold. Glazing method: dipping (using tongs). Thickness: The same. Surface: Clean on both. The difference: Clay wall thickness. The one on the right was cast much thinner so the glaze took a lot longer to dry. 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. But drying needs to occur quickly with glazes containing no gum hardener - that is only possible when the body has enough porosity to absorb all the water quickly. Otherwise, cracks appear - and they become crawls during firing. A complicating factor is that stain and/or zircon additions make an already crawl-susceptible glaze even worse. One or a combination of the following can be done to minimize crawling on even very thin-walled pieces:
-Apply a thinner glaze layer.
-Heat the bisque before dipping.
-Glaze the inside and outside separately (with drying between).
-Deflocculate the glaze to reduce water content.
-Brush or spray on a CMC gummed version in multiple coats.

Crawling in G2934Y Zircopax white glaze: Here are some fixes.

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G2934Y, a variation of the G2934 base, is a good stain matte 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. This base is normally fine as is but when opacified or certain stains are added (especially at significant percentages) it can crawl. This has 10% Zircopax. Even though the glaze layer thickens at the recess of the handle join it is still crawling. We also get this on the insides of mugs where wall and foot meet at a sharp angle. This was initiated because the glaze cracked here during drying. Normally it would heal but the zircon stiffens the melt, making it less mobile. The easiest solution is to adjust the specific gravity of the glaze to 1.44 and flocculate it to thixotropic, this assures that the application is not too thick. Another measure is to add a little CMC gum (by replacing some of the water with gum solution). Lastly, use a blend of tin oxide and Zircopax, as in the G3926C version of the recipe, to opacify it.

5% titanium dioxide in G2934Y matte, G1214Z1 matte, G2926B glossy

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

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

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The body is Plainsman M390. These are commonly used base glazes. The top one is a magnesia matte, next down is a calcia 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.

Emulating a speckled reduction fired stoneware in oxidation

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The body is Plainsman M340S. Cone 6. Left to right: G1214Z1 calciua 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.

Cobalt and iron overglazes bleeding into the matte glaze

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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.

Orange matte glazed porcelain mug

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

What if G2934 fires too glossy, how can you increase matteness?

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Typically the G2934 cone 6 magnesia matte recipe fires with a surface that is too matte for functional ware (with cutlery marking and staining problems). This is intentional - it enables users to blend in a glossy base transparent to tune the degree of matteness. However, we have seen variation in the Ferro Frit 3124, serious enough that a recent production batch of glaze came out glossy (upper left in this picture)! This happened despite a C6DHSC slow cool firing. Shown here is a trial with additions of 4% calcined alumina (upper right) and 6 and 8% (bottom). All of these were too matte (1.5% turned up to be good). Although the slow-cool C6DHSC firing is the likely reason for the opacity here, opacity disruption still turned out to be a factor for stain additions (muting the colors slightly) even in faster cool firings. This is a testament to the critical chemistry balance that produces this matte surface. And the need to have adjustment options when inevitable variation occurs. Of course, it is important to use ultra-fine alumina (e.g. 400 mesh) to assure it will dissolve in the melt.

Links

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