High LOI materials can turn your glaze into Aero chocolate!The smooth surface of this blistering glaze has been ground off to reveal how serious the bubble problem really is. If the body or glaze itself is generating gases of decomposition at the wrong time, and the glaze has too little melt fluidity to pass the bubbles, this can happen. Opacifiers ... more The smooth surface of this blistering glaze has been ground off to reveal how serious the bubble problem really is. If the body or glaze itself is generating gases of decomposition at the wrong time, and the glaze has too little melt fluidity to pass the bubbles, this can happen. Opacifiers (especially zircon) and stains can reduce melt fluidity dramatically. Even glossy smooth glazes can have bubbles lurking just below the surface, wear and tear on a piece can open them up, creating micro holes that roughen the surface. Context: LOI, Glaze Bubbles Thursday 8th August 2019 
A runny glaze is blistering on the inside of a large bowlThe glaze is running down on the inside, so it has a high melt fluidity. "High melt fluidity" is another way of saying that it is being over fired to get the visual effect. It is percolating at top temperature (during the temperature-hold period), forming bubbles. There is enough surface tension to ... more The glaze is running down on the inside, so it has a high melt fluidity. "High melt fluidity" is another way of saying that it is being over fired to get the visual effect. It is percolating at top temperature (during the temperature-hold period), forming bubbles. There is enough surface tension to maintain them all the way down to the body, and for as long as the temperature is held. To break the bubbles and heal up after them the kiln needs to be cooled to a point where decreasing melt fluidity can overcome the surface tension. The hold temperature needs to be high enough that the glaze is still fluid enough to run in and and heal the residual craters. A typical drop temperature is 100F. Context: Melt Fluidity, Fluid Melt Glazes, Blisters, Glaze Blisters Thursday 8th August 2019 
Two batches of EPK tested quite differently in 2018The kaolin has been mixed 70:30 with nepheline syenite to create a body that matures below cone 6. This enables comparing the degree to which the two kaolins influence vitrification. The darker one is more plastic, has higher drying shrinkage more soluble salts. These dry test bars were fired at ... more The kaolin has been mixed 70:30 with nepheline syenite to create a body that matures below cone 6. This enables comparing the degree to which the two kaolins influence vitrification. The darker one is more plastic, has higher drying shrinkage more soluble salts. These dry test bars were fired at cone 4-8 and had similar fired shrinkages and porosities. However the one on the right fired whiter at all temperatures. Context: EP Kaolin Sunday 4th August 2019 
White majolica bases have very, very low melt fluidityThis is Linda Arbuckle's base recipe (66% frit 3124, 23% feldspar, 13 kaolin/bentonite, 9 zircon, 4 tin oxide mixed to 1.62 specific gravity). It is fired at cone 05 creating a super gloss. This is applied very, very thickly (double the thickness of what a stoneware glaze would be). Yet notice how ... more This is Linda Arbuckle's base recipe (66% frit 3124, 23% feldspar, 13 kaolin/bentonite, 9 zircon, 4 tin oxide mixed to 1.62 specific gravity). It is fired at cone 05 creating a super gloss. This is applied very, very thickly (double the thickness of what a stoneware glaze would be). Yet notice how the air-vent holes did not heal during firing, these would have filled in easily had this been on stoneware or porcelain. For this reason, majolica glazes must be applied to ware not having any abrupt concave contours (this also happened on the handle-joins and where foot-meets-wall inside). Bisque must be clean to assure good adherence. Any glaze coverage issues must be repaired carefully before firing. This piece was bisque fired at cone 06, that is the reason for the air vent holes. Had it been bisque fired at cone 04 (higher than the glaze firing) these would not have occurred. However that would have extended the dip-time, this one was held under 10 seconds whereas a cone 04 bisque would have required 20 seconds. Context: For even coverage white majolica glazes must be applied by dipping, Melt Fluidity, Majolica Wednesday 31st July 2019 
For even coverage white majolica glazes must be applied by dippingThe mug on the left has three coats of Spectrum majolica base, painted on by brush from a pint jar. Drying was required after doing the inside coats, so the total glazing time was several hours. The glaze layer is way too thin and it is not even at all! The one on the right was dipped in a 5 gallon ... more The mug on the left has three coats of Spectrum majolica base, painted on by brush from a pint jar. Drying was required after doing the inside coats, so the total glazing time was several hours. The glaze layer is way too thin and it is not even at all! The one on the right was dipped in a 5 gallon bucket-full of Arbuckle white (that was weighed out according to a recipe and slurried at 1.62 specific gravity). It took seconds to dip-apply, the thickness is good and I got very even coverage. As is obvious, it makes sense to make your own base white. Then, you can decorate using the overglaze colors (I use the Spectrum Majolica series). Another advantage of making your own white is that you can splurge on the amount of opacifier (in this case 9% zircon and 4% tin oxide), to achieve maximum whiteness and opacity. And, you can proportion a mix of two frits (having higher and lower thermal expansion) to fine tune the fit with the body (a big issue at low fire). Context: White majolica bases have very, very low melt fluidity, Majolica, Brushing Glazes, Base-Coat Dipping Glazes Wednesday 31st July 2019 
BEWARE of leaving outsides of functional ware unglazedThis mug is made from the strongest porcelain I have, it is so vitreous that the bare fired surface does not even coffee-stain. So I glazed it only on the inside. That created a time-bomb waiting for hot coffee! Three others did exactly the same. Four other mugs glazed on the outside were fine. Why? ... more This mug is made from the strongest porcelain I have, it is so vitreous that the bare fired surface does not even coffee-stain. So I glazed it only on the inside. That created a time-bomb waiting for hot coffee! Three others did exactly the same. Four other mugs glazed on the outside were fine. Why? Glazes need to have a lower thermal expansion than the body so they do not craze over time. When ware is glazed inside and the compressive forces the glaze finds itself under keep it crack free and also significantly strengthens the piece (like pre-stressed concrete). But here there is no outside glaze to be counteract the inside one pushing outward. When suddenly heated it pushes even harder. Structural weak points, outside surface imperfections or pronounced contour or thickness changes provide crack-initiation-points to relieve the stress. The only way to make this inside-only-glazed technique work is carefully tuning the thermal expansion of the inside glaze. That means a lot of testing and a lot of broken pieces. Context: Glaze Compression Wednesday 24th July 2019 
What is the temperature difference between these two cones?Four degrees F. I was consistently getting the cone on the left using a custom programmed firing schedule to 2204F. However Orton recommends that the tip of the self supporting cone should be even with the top of the base, not the bottom. So I changed the temperature to 2200F and got the cone on the right. Context: Interpreting Orton Cones, Cones Wednesday 24th July 2019 
Cone 6 Fluid-Melt Transparent Glaze - Jackpot!In 2015 we did a project comparing common cone 6 fluid-melt base glazes, picked a favourite (Panama Blue) and fixed it's slurry issues and crazing. Fluid-melts almost run off ware when applied thick, but they host stains & opacifiers to produce super-gloss, super-brights. That recipe, G3806C, has ... more In 2015 we did a project comparing common cone 6 fluid-melt base glazes, picked a favourite (Panama Blue) and fixed it's slurry issues and crazing. Fluid-melts almost run off ware when applied thick, but they host stains & opacifiers to produce super-gloss, super-brights. That recipe, G3806C, has been among the most popular pages on our site. In 2019 we moved the thermal expansion much lower (from 7.3 to an incredible 5.7, it can survive a 325F-to-icewater test on our toughest-to-fit porcelain). The best variation, G3806N, is fluidity-controllable (by adjusting kaolin content), durable (double the Al2O3/SiO2 of other common ones) and employs lithium, strontium and magnesia frits. This version, G3806E, has 4% added copper oxide and sources SrO, Li2O and MgO from strontium carbonate, spodumene and Frit 3249. Follow the link here to see the entire history of this development effort (beware, there are multiple pages, each with many columns). Context: 2019 Development of G3806 melt-fluid low-expansion clear base glaze Monday 22nd July 2019 
Sourcing Li2O from spodumene instead of lithium carbonateLithium carbonate is now incredibly expensive. The glaze on the left employs it. But I was able to source the Li2O from spodumene instead. Spodumene has a complex chemistry, but the oxides that it contains (other than Li2O) are those common to glazes anyway. Using my account at insight-live.com, I ... more Lithium carbonate is now incredibly expensive. The glaze on the left employs it. But I was able to source the Li2O from spodumene instead. Spodumene has a complex chemistry, but the oxides that it contains (other than Li2O) are those common to glazes anyway. Using my account at insight-live.com, I did the calculations and got a pretty good match in the formulas (lower section in the green boxes). Then I made 10 gram balls and did a melt flow test at 2200F (notice the long crystals in the pools of glass below the runways). And I glazed buff stoneware test (below the flow tests). Not surprisingly, this glaze is very runny. That's why the tiny yellow crystals grow during cooling, they produce the gold effect this recipe is known for. The spodumene version is very similar, perhaps better. Notice my calculated cost: $15.47/kg vs. $8.12/kg. Context: Spodumene, Lithium Carbonate Monday 22nd July 2019 
Tin Oxide is expensive, do not waste it by not mixing wellThis is a cone 04 glaze on a terra cotta body. Two 300-gram test batches were made. Both have 5% tin oxide added. The one on the left was high-speed propeller-mixed for 10 seconds on a closed container. That was not enough, small agglomerates appear as white specks floating in the glass. The one on ... more This is a cone 04 glaze on a terra cotta body. Two 300-gram test batches were made. Both have 5% tin oxide added. The one on the left was high-speed propeller-mixed for 10 seconds on a closed container. That was not enough, small agglomerates appear as white specks floating in the glass. The one on the right was mixed for 60 seconds. Now the tin particles, which are incredibly small, have been dispersed and can do their job of opacifying the glaze. Notice that 5% is not quite enough, more is needed. Context: Tin Oxide, Propeller Mixer, Opacifier, Opacification Saturday 20th July 2019 | Contact Us:
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