Modification Date: 2018-05-12 13:20:15
A base transparent glaze recipe created by Tony Hansen for Plainsman Clays, it fires high gloss and ultra clear with low melt mobility.
|Ferro Frit 3134||25.4||23.1%|
|Rate (F)||Temp (F)||Hold (Min)||Step|
This is an adjustment to an original recipe named Perkins Studio Clear (it contains alot more SiO2 and uses a frit instead of Gerstley Borate as the boron source). It is just as shiny and transparent, has a less fluid melt and will be harder and for stable. With this change this is a really stunning transparent glaze. We have found that it will even tolerate 5% more silica than what is shown here, yet still smooths out well.
In our testing this glaze survives a 300F oven-to-icewater test without crazing on Plainsman M370 (25-Porcelain using Nepheline, Tile#6 Kaolin, silica and Old Hickory ball clay. This glaze is less affected than the original when the application is too thick (there are minimal bubbles and crazing).
This recipe has very good suspension and application properties if you mix it as we say. It dries hard, does not crack during drying, does not settle hard in the bucket and applies evenly and dries quickly.
To prepare it for use, target a specific gravity of 1.43-1.44 (equal weights of water and powder should produce a value slightly above this, fine tune it down by adding more water). Then add a flocculant (epsom salts, vinegar) to make it creamy. See the thixotropy glossary entry link below for more information on doing this.
Screen through 80 mesh (tiny wollastonite agglomerates are likely, they will not break down without screening).
Plainsmanclays.com makes this recipe as a premixed powder. The glazes section on their site has additional guidelines on this use of this recipe.
If this crazes see the links below.
The clay is Plainsman M390. The inside glaze is G2926B cone 6 base transparent. The outside glaze is the same recipe but with 4% added red iron oxide (screened to 80 mesh). A white engobe (L3954B) has been applied inside and outside down to the midway point (done by pour-filling the leather-hard piece, then pouring it out and pressing it lip-down into the engobe). The incised design was done after the piece had stiffened (after the engobe application). The inside clear glaze was poured in and out, then the lip dipped. Wax resist was applied to the top inch inside an up over the lip. A sharp knife was then used to cut away the glaze from the outside of the lip (at a 45 degree angle) and remaining glaze from there down the outside was scraped off with a fettling knife. The mug lip was then dipped into the outside glaze, the mug quickly turn over and pressing down into the same glaze. Finally, any drips of the glaze on the wax were sponged off and the foot ring cleaned off.
Fired at cone 6. A melt fluidity comparison (behind) shows the G3808A clear base is much more fluid. While G2926B is a very good crystal clear transparent by itself (and with some colorants), with 2% added copper oxide it is unable to heal all the surface defects (caused by the escaping gases as the copper decomposes). The G3808A, by itself, is too fluid (to the point it will run down off the ware onto the shelf during firing). But that fluidity is needed to develop the copper blue effect (actually, this one is a little more fluid that it needs to be). Because copper blue and green glazes need fluid bases, strategies are needed to avoid them running off the ware. That normally involves thinner application, use on more horizontal surfaces or away from the lower parts of verticals.
A transparent glazed. It is a made from Plainsman Polar Ice in 2014 (a New Zealand kaolin based porcelain) and fired to cone 6 with G2926B clear glaze. 5% Mason 6306 teal blue stain was added to the clay, then this was wedged only a few times. The piece was thrown, then trimmed on the outside at the leather hard stage and sanded on the inside when dry.
Melt fluidity test showing Perkins Studio clear recipe original (left) and a reformulated version that sources the boron from Ferro Frit 3134 instead of Gerstley Borate (right). The later is less amber in color (indicating less iron) and it melts to very close to the same degree.
The green boxes show cone 6 Perkins Studio Clear (left) beside an adjustment to it that I am working on (right). I am logged in to my account at insight-live.com. In the recipe on the right, code-numbered G2926A, I am using the calculation tools it provides to substitute Frit 3134 for Gerstley Borate (while maintaining the oxide chemistry). A melt flow comparison of the two (bottom left) shows that the GB version has an amber coloration (from its iron) and that it flows a little more (it has already dripped off). The flow test on the upper left shows G2926A flowing beside PGF1 transparent (a tableware glaze used in industry). Its extra flow indicates that it is too fluid, it can accept some silica. This is very good news because the more silica any glaze can accept the harder, more stable and lower expansion it will be. You might be surprised how much it took, yet still melts to a crystal clear. See the article to find out.
The flow on the left is an adjusted Perkins Frit Clear (we substituted frit for Gerstley Borate). It is a cone 6 transparent that appeared to work well. However it did not survive a 300F oven-to-icewater test without crazing on Plainsman M370. The amount of flow (which increases a little in the frit version) indicates that it is plenty fluid enough to accept some silica. So we added 10% (that is the flow on the right). Now it survives the thermal shock test and still fires absolutely crystal clear.
These are Mason stains added to cone 6 G2926B clear liner base glaze. Notice that the chrome tin maroon 6006 does not develop as well as the G2916F glossy base recipe. The 6020 manganese alumina pink is also not developing. 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.
This is Plainsman translucent Polar Ice firing at cone 6 with a transparent base glaze. Made by Tony Hansen in 2014.
If this happens you need to screen it. There is nothing unusual in the recipe, this can happen to any glaze that contains frits or other slightly soluble materials.
The body is Plainsman M332, a coarse particled brown to red burning cone 6 body. With the G2926B transparent cone 6 glaze (left) and the GA6-A Alberta Slip base (right). The latter brings out the color of the body much better, the former is milky, bubbly and yucky!
The outer green glaze on these cone 6 porcelain mugs has a high melt fluidity. The liner glaze on the lower one, G2926B, is high gloss but not highly melt fluid. Notice that it forms a fairly crisp boundary with the outer glaze at the lip of the mug. The upper liner is G3806C, a fluid melt high gloss clear. The outer and inner glazes bleed together completely forming a very fuzzy boundary.
These two mugs are the same dark burning stoneware (Plainsman M390). They have the same clear glaze, G2926B. They are fired to the same temperature in the same firing schedule. But the glaze on the left has 4% added iron oxide. On a light-burning body the iron changes the otherwise transparent glass to amber colored (with speckle). But on this dark burning clay it appears transparent. But amazingly, the bubble clouds are gone. We have not tested further to find the minimum amount of iron needed for this effect.
Potters often store glazes for long periods so tiny spherical precipitate particles can form. These were found in a months-old bucket of G2926B (M370 clear) cone 6 clear glaze (about 2 gallons). These can appear over time, depending on factors like temperature, electrolytes in your water or solubility in the materials (likely, the frit is slightly soluble). The glaze slurry should be screened periodically (or immediately if you note the particles when glazing a piece). This is an 80 mesh screen. Note the brush, using one of these gets the glaze through the screen much quicker than using a rubber spatula.
These cone 6 porcelain mugs are hybrid. Three coats of a commercial glaze painted on outside (Amaco PC-30) and my own liner glaze poured in and out on the inside (G2926B). When commercial glazes (made by one company) fit a stoneware or porcelain (made by another company), without crazing or shivering, it is purely an accident! So use them on the outside. But for inside food surfaces make or mix your own. When you know the recipe you can tune the thermal expansion. And the degree of melt. And the application properties. And you can use quality materials to source a balanced chemistry. The place to start understanding your glazes, organize testing and development and document everything is an account at Insight-live.com.
These are 10 gram balls of four different common cone 6 clear glazes fired to 1800F (bisque temperature). How dense are they? I measured the porosity (by weighing, soaking, weighing again): G2934 cone 6 matte - 21%. G2926B cone 6 glossy - 0%. G2916F cone 6 glossy - 8%. G1215U cone 6 low expansion glossy - 2%. The implications: G2926B is already sealing the surface at 1800F. If the gases of decomposing organics in the body have not been fully expelled, how are they going to get through it? Pressure will build and as soon as the glaze is fluid enough, they will enter it en masse. Or, they will concentrate at discontinuities and defects in the surface and create pinholes and blisters. Clearly, ware needs to be bisque fired higher than 1800F.
The porcelain mug on the left is fired to cone 6 with G2926B clear glossy glaze. This recipe only contains 25% boron frit (0.33 molar of B2O3). Yet the mug on the right (the same clay and glaze) is only fired to cone 02 yet the same glaze is already well melted! What does this mean? Industry avoids high boron glazes (they consider 0.33 to be high boron) because this early melting behavior means gases cannot clear before the glaze starts to melt (causing surface defects). For this reason, fast fire glazes melt much later. Yet many middle temperature reactive glazes in use by potters have double the amount of B2O3 that this glaze has!
G2922B is a cone 6 clear glaze that started as a well-known recipe "Perkins Studio Clear". We substituted Gerstley Borate with a frit (while maintaining the chemistry) and then noted that the glaze was highly fluid. Since I wanted to keep its thermal expansion as low as possible, I added 10% silica. 2926B shows that it is very well tolerated. Then I added 5% more (2926D) and 10% more (2926E which is still very glossy). That means that E represents a full 20% silica addition! SiO2 has no real downsides in any well melted glossy glaze, it hardens, stabilizes and lowers expansion.
10 grams balls of these three glazes were fired to cone 6 on porcelain tiles. Notice the difference in the degree of melt? Why? You could just say glaze 2 has more frit and feldspar. But we can dig deeper. Compare the yellow and blue numbers: Glaze 2 and 3 have much more B2O3 (boron, the key flux for cone 6 glazes) and lower SiO2 (silica, it is refractory). That is a better explanation for the much greater melting. But notice that glaze 2 and 3 have the same chemistry, but 3 is melting more? Why? Because of the mineralogy of Gerstley Borate. It yields its boron earlier in the firing, getting the melting started sooner. Notice it also stains the glaze amber, it is not as pure as the frit. Notice the calculated thermal expansion: That greater melting came at a cost, the thermal expansion is alot higher so 2 and 3 glaze will be more likely to craze than G2926B (number 1).
Boron (B2O3) is like silica, but it is also a flux. Frits and Gerstley Borate supply it to glazes. In this test, I increased the amount of boron from 0.33 to 0.40 (using the chemistry tools in my insight-live.com account). I was sure that this would make the glaze melt more and have less of a tendency to craze. But as these melt flow tests (10 gram balls melted on porcelain tiles) show, that did not happen. Why? I am guessing that to get the effect B2O3 has to be substituted, molecule for molecule for SiO2 (not just added to the glaze).
We are comparing the degree of melt fluidity (10 gram balls melted down onto a tile) between two base clear glazes fired to cone 6 (top) and cone 1 (bottom). Left: G2926B clear boron-fluxed (0.33 molar) clear base glaze sold by Plainsman Clays. Right: G3814 zinc-fluxed (0.19 molar) clear base. Two things are clear: Zinc is a powerful flux (it only takes 5% in the recipe to yield the 0.19 molar). Zinc melts late: Notice that the boron-fluxed glaze is already flowing well at cone 1, whereas the zinc one has not even started. This is very good for fast fire because the unmelted glaze will pass more gases of decomposition from the body before it melts, producing fewer glaze defects.
These melt-flow and ball-melt tests compare 6 unconventionally fluxed glazes with a traditional cone 6 moderately boron fluxed (+soda/calcia/magnesia) base (far left Plainsman G2926B). The objective is to achieve higher melt fluidity for a more brilliant surface and for more reactive response with colorant and variegator additions (with awareness of downsides of this). Classified by most active fluxes they are: G3814 - Moderate zinc, no boron G2938 - High-soda+lithia+strontium G3808 - High boron+soda (Gerstley Borate based) G3808A - 3808 chemistry sourced from frits G3813 - Boron+zinc+lithia G3806B - Soda+zinc+strontium+boron (mixed oxide effect) This series of tests was done to choose a recipe, that while more fluid, will have a minimum of the problems associated with such (e.g. crazing, blistering, excessive running, susceptibility to leaching). As a final step the recipe will be adjusted as needed. We eventually chose G3806B and further modified it to reduce the thermal expansion.
These two glazes are both brilliant glass-like super-transparents. But on this high-iron stoneware only one is working. Why? G3806C (on the outside of the piece on the left) melts more, it is fluid and much more runny. This melt fluidity gives it the capacity to pass the micro-bubbles generated by the body during firing. G2926B (right) works great on porcelain but it cannot clear the clouds of micro-bubbles coming out of this body. Even the glassy smooth surface has been affected. The moral: You need two base transparents in which to put your colors, opacifiers and variegators. Reactive glazes need melt fluidity to develop those interesting surfaces. But they are more tricky to use and do not fire as durable.
These porcelain mugs were decorated with the same underglazes (applied at leather hard), then bisque fired, dipped in clear glaze and fired to cone 6. While the G2926B clear glaze (left) is a durable and a great super glossy transparent for general use, its melt fluidity is not enough to clear the micro-bubbles generated by the underglazes. G3806C (right) has a more fluid melt and is a much better choice to transmit the underglaze colors. But I still applied G2926B on the inside of the mug on the right, it has a lower thermal expansion and is less likely to craze.
Typical zero-boron high temperature glazes will not soften in a 1500F decal firing. But low temperature glazes will (especially those high in boron). Even middle temperature ones can soften. G3806C, for example, is reactive and fluid, it certainly will. Even G2926B, which has high Al2O3 and SiO2, has enough boron to soften and sometimes create tiny pits. In serious cases they can bubble like the mug on the right. Why? Steam. It was in use and had been absorbing water in the months since it was first glaze fired at cone 03. The one on the left was not used, but it did have some time to absorb water from the air, it is showing tiny pits in the surface. Even if moisture is not present, low fire bodies especially may still have some gases of decomposition to affect the glaze. One more thing: Fire the decals at the recommended temperature, often cone 022.
These two pieces are fired at cone 6. The base transparent glaze is the same (G2926B Plainsman transparent). The amount of encapsulated red stain is the same (11% Mason 6021 Dark Red). But two things are different. Number 1: 2% zircon has been added to the upper glaze. The stain manufacturers recommend this, saying that it makes for brighter color. However that is not what we see here. What we do see is the particles of unmelting zircon are acting as seed and collection points for the bubbles (the larger ones produced are escaping). Number 2: The firing schedule. The top one has been fired to approach cone 6 and 100F/hr, held for five minutes at 2200F (cone 6 as verified in our kiln by cones), dropped quickly to 2100F and held for 30 minutes.
This is G2926B clear cone 6 glaze deflocculated with Darvan. Because the Darvan is thinning it, 2.5kg of glaze powder is suspended in only 1100g (1100ml) of water (half the normal amount). While the slurry in the bucket flows well and appears like it should work, a one-second dip produces twice the desired thickness. It dries slowly and it is very difficult to prevent runs. The lesson: Make sure the specific gravity (SG) of your glazes is right. What should the SG be? Measure it when your glaze is working well. Or take note of it in instructions that come with the recipes you use. For bisque ware: 1.43-1.45 with a flocculant (like Vinegar or powdered Epsom Salts) added to gel the slurry slightly.
The outside glaze is a copper blue, but that is not the one we are interested in here. It is the clear glaze on the insides of these two identical cone 6 porcelain mugs. Why add such a small amount of zircon to it? It is not being added to opacify, it is being added to toughen the surface and reduce the thermal expansion.The presence of the 2% zircon has not affected the gloss or transparency of the glaze on the right. However the 3% on the left has opacified it just slightly and made the surface a little silky. So that is too much for this glaze (although it might be OK if the melt fluidity was higher). So, if you are interested in the most functional possible surface, consider a 2% zircon addition to your transparent.
Out Bound Links
A transcript of a presentation at the 3rd Whitewares conference at Alfred University in the spring of 2000 by Richard Eppler.
A recipe sourcing high MgO (from Ferro Frit 3249) to produce a low expansion glass resistant to crazing on lower silica porcelains.
2003-12-18 - The original impetus to create this recipe was to reduce the thermal expansion of the G1214M and G1215W recipes to work better on porcelains (the othe...
Substitute for low expansion cone 6 G1215U, this sources MgO from talc instead of a frit
2013-05-29 - During 2013 we saw problems with the G1215U low expansion glossy that we had been recommending from some years. It was not melting to a completely ult...
Incorporates some MgO (at the expense of CaO, KNaO) to reduce the thermal expansion of G1214M 5x20 glaze.
2014-01-02 - This glaze adjusts G1214M 5x20 to reduce its thermal expansion (incorporating MgO). It goes in the same direction as G1215U but does not add as much M...
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.
2014-03-26 - This glaze is a cone 6 dolomite matte. It is the product of a series of tests to determine the best levels of SiO2 and Al2O3 in a boron fluxed (but no...
A comparison of the weights of equal volumes of a given liquid and water. Water has a specific gravity of 1.0. A ceramic slurry with a specific gravity of 1.8 is thus 1.8 times heavier than water. The best way to measure specific gravity is to weigh a container and record its weight, then weigh the ...
A base fluid-melt glaze recipe developed by Tony Hansen. With colorant additions it forms reactive melts that variegate and run. It is more resistant to crazing than others.
2015-09-30 - This is a test to publish Insight-live data here. Many more recipes coming soon.
A step-by-step process to put a liner glaze in a mug that meets in a perfect line with the outside glaze at the rim.
Moving a cone 10 high temperature glaze down to cone 5-6 can require major surgery on the recipe or the transplantation of the color and surface mechanisms into a similar cone 6 base glaze.
Knowing about thixotropy will enable you to mix a glaze that stays in suspension much better. It does not drip alot when a piece is draining. It goes on evenly, of adequate thickness and does not run. It dries quickly (on porous bisque) and is just much nicer to use. The secret to all of this is not...
In Bound Links
Break your addiction to online recipes that don't work. Get control. Learn why glazes fire as they do. Why each material is used. Some chemistry. How to create perfect dipping and drying properties. Be empowered. Adjust recipes with issues rather than sta
This Plainsman Cone 6 Ravenscrag Slip base is just the pure material with 20% added frit to make it melt to a glossy natural clear.
2003-07-21 - This is the base cone 6 Ravenscrag recipe, it fires as a transparent glossy. It has an addition of the most common North American borate frit, enough ...
Crystal clear industrial dinnerware glaze
2013-09-26 - This is an industrial tableware glaze recommended by tech support at Fusion Frits. It not only fires hard and crystal clear but has outstanding suspen...
Kill the Gerstley borate. Fix the crazing. These are some of the most common things people do using Insight software. This can be a fantastic clear glaze if you adjust it to stop the crazing and repla...
It is better to understand and have control of one good base glaze than be at the mercy of dozens of imported recipes that do not work. There is a lot more to being a good glaze than fired appearance.
A very low thermal expansion version of the G2926B base recipe. Use this if the B versions crazes on your clay body.- 2015-10-01
A low expansion transparent cone 6 glaze for Plainsman P300 (and other kaolin only porcelains difficult to fit glazes to)
2016-08-24 - We have been recommending G2926J, an adjustment to G2926B to lower its thermal expansion. However J does not have as smooth of a surface or is as crys...
The term 'limit formula' historically has typically referred to efforts to establish absolute ranges for mixtures of oxides that melt well at an intended temperature and are not in sufficient excess to cause defects. These formulas typically show ranges for each oxide commonly used in a specific gla...
A base glaze is one having no opacifiers, variegators or colorants. Thus it should be transparent if glossy and translucent if matte. Developing or adapting a base glaze for your ware is a very important first step in developing a manufacturing process that produces good quality. In fact, from a qua...
A fully transparent glaze is simply one that does not have opacity. But there are degrees of transparency. For example, if a glaze is matte it will show the color of underlying body and decoration, but these will be muted (so it is actually translucent). Completely transparent glazes look like a gla...
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By Tony Hansen