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Technical Tips Blog
Are manganese speckled clay bodies a toxicity hazard?
Before jumping to conclusions consider all the factors that relate. This is M340S, it is fired at cone 6. That temperature is a "sweet spot" for this effect, high enough for the particles to bleed and low enough they do not bloat the body. Such bodies contain only about 0.2% of 60-80 mesh granular manganese (compare this to many glazes that employ 5% powdered manganese as a colorant). Further, the vast majority of the manganese particles are encapsulated within the clay matrix. The tiny percentage exposed at the body surface are under the glaze. It is not the manganese particles themselves that expose at the glaze surface. Rather particle surfaces that contact the underside of the glaze bleed out into it from below, doing so as a function the glaze thickness, opacity and melt fluidity. Thus, food contact with a glass surface having isolated manganese-pigmented regions is not at all the same thing as with raw manganese metal. Consider also that the total area of manganese-stained glass on a functional surface is extremely small for this effect.
Tuesday 27th April 2021
Two cone 10R bamboo glazes. One stains, one does not. Why?
These mugs are Plainsman H443. The bamboo glaze on the left (A) has 3.5% rutile and 10% zircopax added to the base G2571A dolomite matte. The one on the right (B) has the same addition but in a base having slightly less MgO and slightly more KNaO. B stains badly (as can be seen from the felt marker residue that could not be removed using lacquer thinner). Why does A stain only slightly? It has an additional 4% Gerstley Borate (GB). GB is a powerful flux that develops the glass better, making the surface more silky. The differences in the recipe provide another advantage: (A) has a much lower thermal expansion and is less likely to craze.
Tuesday 27th April 2021
Should you expect to vitrify terra cotta?
These were cast by Anna Lisovskaya, they are fired at cone 03. They are supposed to fit into hexagonal welded frames, but during firing many of them warp enough to fit poorly. Why? The color differences are most obvious here. With that color associates a firing shrinkage difference, the darker ones shrink significantly more. Something less obvious: the sides against the elements receive direct radiant heat, so they shrink more, turning a perfect hexagon into an imperfect one. Terra cotta clays are volatile, that means their approach to maximum density during heat-up, accompanied by shrinkage, happens across a narrow temperature range. Accurate and even firing are paramount. In a radiant-heat electric kiln this can be very difficult. Two approaches could work here: Fire at a lower temperature, perhaps cone 04. Or, greatly slow rate-of-rise for the last 100F, perhaps over several hours.
Thursday 22nd April 2021
What is the difference between fahrenheit degrees and degrees fahrenheit?
This is important to understand that when looking at our firing schedule charts. “Degrees Fahrenheit” is a measure of the temperature of something. For example, 212F is the boiling point of water (the equivalent of 100C). "Fahrenheit degrees" are the divisions on the thermometer, there are 170 of them between the freezing and boiling point of water, for example (32-212, while there are 100 celcius degrees for the same span). "Fahrenheit degrees" are thus measures of change-in-temperature, not what the temperature is. In firing schedules, that is what we are talking about, how many degrees should the kiln rise during each step.
Context: Firing Schedule
Wednesday 21st April 2021
Crawling in G2934Y zircon white glaze: There are simple fixes
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. Crawling can also happen on the insides of mugs, where wall and foot meet at a sharp angle. This happens, both because the glaze cracked here during drying and because the zircon stiffens the melt, making it less mobile. Rounding such contours will help. Even better, adjust the glaze recipe so it shrinks a little less on drying (by trading 5% of the raw kaolin for calcined). Adding a little CMC gum (e.g. 0.1-0.2%) will make it adhere better.
Saturday 20th March 2021
Blisters in a highly melt-fluid cone 6 sculpture glaze
Why are these happening (on this piece by Paul Briggs)? It is not completely clear. The glaze has plenty of carbonates, including copper, enough for over 20% LOI. But these normally produce high populations of small blisters, this is the opposite. The melt appears to have enough surface tension that the bubbles survive and endure top-temperature-soaking. And they don't pop until the temperature has dropped so far that insufficient melt-mobility remains to heal them. The glaze has an unconventionally low SiO2 content, that makes it flow vigorously, well enough that the melt is moving and collecting in surface contours. The glaze recipe is quite unconventional, any effort to "improve" its adherence to limits would likely lose the visual aesthetic. A drop-and-hold firing schedule is likely the key to alleviating this.
Friday 19th March 2021
Cone 6 oil-spot glaze effect, what works and does not work?
Simulating a white-on-black oil-spot effect at cone 6 oxidation proved to be a matter of repeated testing (that got me past some misconceptions). Stopping to think about the results at each step and keeping a good audit trail with pictures, in my account at insight-live.com, really helped. I had three black glazes: G2934BL satin (G2934 with black stain), G2926BB super-gloss (G2926B with black stain) and G3914A Alberta Slip black. Going on a hunch, I mixed up a bucket of the G3914A first (with some gum to help it survive second-coating without lifting). Rather than just try any white, I created G3912A by substituting as much CaO and MgO as possible for SrO in the G2934Y base. I later learned this to be an error, SrO reduces the surface tension, I should have used MgO (the G2934Y is a high-MgO glaze so it would have been fine as-is)! As you can see on the far right, this white still worked (at cone 5, 6, 7, 8). Why? There is another factor even more important. The effect only works on the Alberta Slip black. But its LOI is not higher than the others. And it worked even after ball milling. So I need to continue to work on this to learn more about why this works.
Friday 19th March 2021
Casting plates, is it practical?
No. Because you will face a whole array of problems. This is the first, poor mold release, or more correct, impossible mold release! Plates will be too thin walled. If you cast them longer wall thickness will be uneven. Edges will crack like this (because of poor plasticity). They will warp during drying. They will lack dry strength for handling. They will warp during firing. You won't be able to get a good rim. You won't be able to cast a foot ring without an indent showing on the inside. Note here that another issue is at play: The clay is either not plastic enough to cut cleanly at the rim, without tearing. Or, it is being cut too late or with a dull knife. These tears provide places for cracks to initiate. Plates are much better made using the jiggering, ram pressing or dust pressing processes. Or by throwing them on plaster batts.
Tuesday 16th March 2021
Drip glazing and bare outsides: Deceptively difficult.
Why? Glaze fit. These are available on Aliexpress (as Drip Pottery) and they are made by a manufacturer that has close control of body maturity (and thus strength) and a dilatometer to precisely match the thermal expansion of the glaze. The glaze has to fit better than normal because of the absence of an outside glaze. Too low an expansion and it's compression (outward pressure) will fracture body (because these are thin-walled pieces). Too high and it will craze. And that thick glaze? It will shiver or craze with far less forgiveness than a thin layer. And how did they get the glaze on this thick? They deflocculated it, up to 1.7 or more, glazed the inside, let it dry, then glazed the outside. These pieces are a visual and technical achievement. If you are a potter you had best think twice before attempting the same.
Monday 15th March 2021
How to make an incredibly white engobe for terra cotta
I found seven secrets with recipe, process, glaze and firing. 1. A lot of Zircopax, in this case 20% (for whiteness, opacity). 2. The whitest burning materials: New Zealand Halloysite as the kaolin and nepheline syenite as the feldspar. 3. 3% Veegum to gel the slurry (enabling low specific gravity for thin and even coating). 4. The recipe, L3685Z2, has 55% kaolin, that will certainly produce drying cracks. But 1% CMC gum stops that and makes it brushable. It even works on on bisque, I pour-applied it to the insides of these two slip-cast pieces, it drained to perfectly even coverage (in a very thin layer). 5. A terra cotta casting/throwing body to fit the engobe to (has the same fired shrinkage at a target temperature, e.g. cone 04): Initially I am using L4170B. 6. A clear glaze that fits and is transparent: Notice how much whiter the left one is, G3879. At the same thickness as the G1916Q on the right, it is more transparent, better transmitting the white of the engobe. 7. The right firing curve: The 04DSDH drop-and-hold schedule for defect free surfaces.
Thursday 11th March 2021