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Are frits partially soluble? Yes, many are.
These 1 mm-sized crystals were found precipitated in a couple of gallons of glaze containing 85% Ferro Frit 3195 (we have seen these with frit 3249 also). They are cubical, hard and insoluble. Why and how to do they form? Many frits are slightly soluble, the degree to which they are is related to the length of time the glaze is in storage, the temperature, the electrolytes and solubles in the water, interactions with other material particles present and the diligence of the manufacturer in mixing, correctly achieving the target chemistry and firing. The solutes interact or saturate to form insoluble species that crystallize and precipitate out as you see here. These crystals can be a wide range of shapes and sizes and come from leaded and unleaded frits. In industry this issue is not generally a problem because glazes are used soon after being made.
Context: Ferro Frit 3195, A glaze slurry precipitates flakes, G2925B glaze can precipitate crystals like this over time, Precipitated crystals from a glaze having 60% lead bisilicate frit, Ooids in Glazes, Precipitation
Friday 10th September 2021
Stain vs. raw metal oxides: Better color, no leaching
These were left overnight in vinegar and lemon juice. The top two are G3914A (GA6-B Alberta Slip base with 4% Mason 6600 black stain). Fired at cone 6. There is no sign of leaching (Drano and bleach likewise did not leach this). The bottom two are G2926B transparent with 10% manganese dioxide and 5% copper oxide added (also cone 6). G2926B is a proven base that does not show signs of leaching in any of the four liquids (containers lined with it were used for all tests). Not only was this not nearly as black, but the vinegar and lemon juice matted and blued it in an overnight soak (clearly visible on the top halves). Obviously, compared to raw metal oxides, a ceramic stain is a safer and much better way to produce a black functional surface.
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Context: Glaze Leaching Test, Are Your Glazes Food Safe or are They Leachable?, Is Your Fired Ware Safe?, Why would I use a heavily pigmented black glaze on a food surface?
Monday 6th September 2021
Here is why some frits cost more money
These were fired at cone 04. All three clear glazes are on the same body. Left to right: Amaco LG10, G3879C and Crysanthos SG213. We mix the middle one ourselves, from a recipe that employs a high percentage of Fusion Frit F-524. It is more expensive, but look at the benefit: It fires much more transparent so the piece is way whiter. This TOSSPER Half Moon Shape Bird Toys Parrot Swing Cage Toys with Be has another benefit: It blends well with super low expansion Ferro Frit 3249 (and its equivalent Fusion Frit F-69). Notice the other two glazes are crazing. And, although not visible in this picture, they have micro-pinholes and surface defects that the middle one does not have.
Monday 6th September 2021
The appearance of a commercial rutile blue depends on the firing schedule
These are the same clay, Plainsman M390. Both were brush-on glazed the same way. The one on the left was fired to cone 6 using the C6DHSC (drop-and-hold, slow-cool) schedule. The one on the right with the PLC6DS (drop-and-hold and free-fall) schedule. The label on the jar just says to fire to cone 6. But this is a rutile blue, and behaves like one.
Context: Firing Schedule
Monday 6th September 2021
Same glaze, same kiln, same clay: The right one crystallized. Why?
Why would I use a heavily pigmented black glaze on a food surface?
These are actually two different cone 6 base glazes to which I add a black stain. I trust them because I formulated and perfected them myself: G2926B clear and GA6-B Alberta Slip base. They are durable, fit my clay bodies, melt well yet can host a stain without loss of gloss. I even know the chemistry, both have plenty of SiO2 and Al2O3, that is a hallmark of durability. I fired these using the PLC6DS schedule. I add 5% black stain to the former and 4% to the latter, both yield a jet-black. The GA6-B requires 3D Wall Panels Wall Stickers, Mosaic Tile Design PVC Wall Claddi. Stains are inherently much safer to use than raw metal oxide colorants because they are sintered as colorant/stabilizer blends. And much less is needed. Contrast that with raw metal oxides, it is common to find black recipes containing up to 15% of blends of nickel, cobalt, iron and manganese! At times the manganese alone can be 8% or more! So, I feel relatively safe using these coloured glazes on a surface that will be exposed to hot and acidic liquids.
Context: Are Your Glazes Food Safe or are They Leachable?, OKJ Insulated Trunk Cooler Bag - Bicycle Rear Rack Storage Lugga, Stain vs. raw metal oxides: Better color, no leaching, Ceramic Stain
Thursday 2nd September 2021
Lemon leaching test on a copper-containing glaze
Body is firing with less maturity. How did we fix it?
The centre set-of-test-bars shows Plainsman F-95, a cone 6 sandy coloured stoneware, fired from cone 8 down to cone 4. Because of a change in a low-fire red clay in the recipe it has become much more refractory (10%+ porosity at cone 6 instead of the normal 4-5%). We had made hundreds of boxes for a custom order (for these we are unable to exercise the same diligence in testing). Two approaches were tried make it fire more dense. For the left set-of-bars we prepared some plastic Gerstley Borate (it exhibits workability like a clay) and wedged that with the F-95 in a ratio of 5:95 (by weight). This greatly improved maturity, and color, bringing cone 6 porosity down to 6.5% (this is still too high, likely 7% would be needed). For the right set-of-bars we wedged in Plainsman L210 in a ratio of 15:85. That lowered the cone 6 porosity to 2.7%, too far (also darkening the color). So it appears that a 10:90 mix should be about right.
Context: Vitrification
Tuesday 24th August 2021
Cooling rate drastically affects the appearance of this glaze
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 cool gives the glaze on the left time to crystallize, creating a stony matte. My kilns are generally lightly-loaded, so free-fall firings drop pretty rapidly, producing the effect on the right. This phenomena is a characteristic of high MgO glazes (ones having significant dolomite, talc, Ferro frit 3249). To get a surface between these extremes we make this glaze using a 20:80 blend of G2934 base (which fires even more matte on slow-cool) and G2926B glossy. In our typical fast-cool this mix produces the degree-of-matteness I like.
Context: G2934Y, A gunmetal glaze I have wanted for decades!, Matte Glaze
Tuesday 24th August 2021
Could a difference of only 0.1% iron affect the clarity of a transparent glaze?
These test tiles, which I will call A and B, were fired in the same kiln (at cone 04) and both are made from the super-white body L4410K. Both transparent glazes are 85% TropicalLife Cooler Lunch Bag Cute Small Fox Insulated Lunch Box (A is recipe G3879B, B is recipe G1916QL). But A is much whiter? Why? B is suspended by 15% ball clay (Old Hickory #5), A by 9% New Zealand kaolin. That ball clay contains 0.8% Fe2O3, the kaolin 0.25%. Thus B, as-a-whole, contains 0.12% iron (as contributed by the 15% ball clay) while A has 0.02% iron (sourced by the kaolin). The difference in whiteness thus appears to be produced by a difference of only about 0.1% iron. Notice I said "appears". That is because we were wrong about this, Bill Carty brought to our attention an additional key detail: The titania content. "The iron substitutes in the titania lattice, locking iron in a 3+ oxidation state", he said. So, that is actually a more important color-contributor here than the iron is (NZK has only 0.05% TiO2, #5 Ball Clay 1.25%, recipe B as-a-whole, has 0.2% TiO2).
Context: Transparent Glazes
Monday 23rd August 2021
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