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Thank you for your patience, support, and patronage. Vancouver Island Pottery Supply has been able to continue to serve our Ceramic Artist on Vancouver Island and the surrounding Islands, with curbside pick up during our long Covid-19 restrictions.
Our Covid numbers in BC have continued to decrease in case numbers. Based on Dr. Henry's recommendations we will be able to have customers in the storefront.
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Technical Tips Blog
A salt glazed sewer pipe junction
Made at Alberta Clay Products in Alberta, Canada about 1960. These are massive. They were hand-constructed. This was fired in a beehive kiln and is on display inside a beehive kiln (a historic site) next to the Plainsman Clays plant. Ceramic glazes are normally slurries of clay, quartz and and fluxes like sodium feldspar, calcium carbonate or dolomite - these are applied to ware before firing. But, in this salt-glazing process, common table salt was literally shovelled into a hole at the top as the kiln reached temperature (about 2350F)! The salt decomposed, separating into sodium vapour and chlorine gas within the chamber and the sodium reacted with the quartz-containing clay to form a durable glaze. Unfortunately the chlorine gas escaped into the air!
Context: Salt glaze beehive kiln beside the Plainsman Clays plant, Salt firing
Tuesday 3rd August 2021
Absolutely jet-black cone 6 engobe on M340
This is the L3954B engobe. But it has 15% Mason 6600 black body stain (instead of the normal 10% Zircopax for white). There is no cover glaze, yet it is durable and absolutely coal black (so a lesser stain % is possible). Lots of information is available for L3954B (including mixing instructions showing exact amounts for water, powder, Darvan). Engobes are tricky to use, follow the link above or below to learn more. L3954B is designed to work on regular Plainsman M340 (this piece), M390 and Coffee Clay, these bodies dry better than porcelains and are much less expensive, coating them with an engobe to get a surface like this makes a lot of sense. This engobe is actually a highly plastic body, but it does not contain enough feldspar to be a porcelain (this is on purpose to match the firing shrinkage of the stonewares).
Context: Mason 6600 Black Stain, L3954B, Polar Ice Porcelain with Body Stains - by Robert Barritz, L3954B engobe page at PlainsmanClays.com, Engobe, Thixotropy
Sunday 1st August 2021
Match the firing shrinkage of an engobe to a terra cotta
These lines plot the firing shrinkages for three versions of L3685Z2 engobe. Notice the terra cotta body I want to match (red line) and the black engobe (green line) do not cross anywhere. That means there is no temperature at which they fit each other (the engobe always has 2% or more firing shrinkage). Notice the L4170B terra cotta fits the white version, Z2, at 2150F (red line crosses blue line, but the body is over-fired by that point). For a fit at my preferred 2000F (cone 02) I need the Z4 engobe to shrink 2% more (a 3% addition of frit 3110 will do that). What about the black Z4? That is the opposite situation, it already contains 5% frit, removing that will drop that green line about 3%, hitting the red line at 2000F (and following it all the way down past 1950 into the cone 04 range). I ignored all of this and used the Z2 white on L4170B, L210 and L215. It looked good on most pieces, but sure enough, it did crack around abrupt contours on some. Of course, this does not assume a thermal-expansion-match of body and engobe.
Context: Can engobes be applied to bisque?, Terra cotta
Wednesday 21st July 2021
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 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.
Context: Manganese Granular, G2934Y glaze on Standard #112 body at cone 6
Monday 19th July 2021
How many degrees between these cone positions?
G2926B using Fusion Frit F-12 instead of Ferro 3134
G2926B is a popular recipe and there has been alarm recently because of the difficulty in getting the Ferro frit. This motivated us to get a supply of the Fusion equivalent, F-12. We have done much testing in glazes and with melt fluidity tests like this and so far it is looking good.
Context: Ferro Frit 3134, Fusion Frit F-12
Thursday 8th July 2021
G2934 using Fusion Frit F-19 instead of Ferro 3124
G2934B is a popular recipe and there has been alarm recently because of the difficulty in getting the Ferro frit. This motivated us to get a supply of the Fusion equivalent, F-19. We have done much testing in glazes and with melt fluidity tests like this and so far it is looking good.
Context: Ferro Frit 3124, Fusion Frit F-19
Thursday 8th July 2021
The same glaze on a buff-burning stoneware and a white porcelain
Both of the bodies are made from materials processed to minus 200 mesh. They were fired side-by-side to cone 6. Both bodies are vitreous, in fact, the buff burning one, Plainsman 3D 325, is more vitreous (the white one is Plainsman Polar Ice). The glaze is G2926B with 10% Mason 6304 stain and 2% Zircopax added. Clearly, iron content in the body heavily affects the development of the color in this glaze. But it is more. Because the 3D is highly vitreous the glaze melt is better able to leach iron from it.
Context: Why is this glaze so different on these two different porcelains?
Friday 2nd July 2021
Pottery made from cremation ash, increasingly popular!
As you can see from the search, this is becoming "a thing". The ash is being incorporated into both clay bodies and glazes. The ash of pets and humans. If you are a potter wondering about doing this here are a few tips. Do testing, better to use up some of the ash for that than have to throw away the ware you make! If the ash has not been ground (likely the case for pets) there will be bone fragments, these won't melt so need to be removed for glazes (by sieving or ball milling). For wedging into the body, testing will be needed (consider the possibility of lime-popping). Be careful to write down your procedure during testing so that production does not bring surprises. While you can add ash to commercial bottled glazes, the percentage will be low. If you make your own dipping glaze, 50% ash should be possible. Do tests without colorants to get a base glaze that is melting well and does not crawl. Add stain powders to test colors, zircon and titanium dioxide to opacify (the latter will variegate more). Color and opacifier additions can introduce crawling, test these well also.
Context: Wood Ash Glaze, Lime Popping
Wednesday 30th June 2021
Is it possible to spot a leaching glaze just by looking at it?
Often the answer is yes. This glaze looks too matte, too metallic, too crystalline. This picture was sent to me by a worried person who had bought it and noticed it discolouring on the inside. The potter may very well have considered this safe just because it was fired to cone 10. It is common among potters to overload glazes with raw metal oxide blends, often 15% or more (e.g. manganese, copper, iron, nickel, cobalt). These percentages cannot be held in solution in the melt as it cools and solidifies, so they precipitate out and crystallize, especially if the glaze is not melting well or has insufficient SiO2. The crystalline forms of these metals might look nice to some people, but the glaze is likely to leach them. It is better to use a ceramic stain to create a black like this, adding it to a stable matte base glaze (one that melts well and has sufficient SiO2 and Al2O3 to create a durable glass). The concept of a limit recipe is helpful in eyeballing recipes for their likelihood of leaching.
Context: Are Your Glazes Food Safe or are They Leachable?, How to Liner-Glaze a Mug, Is Your Fired Ware Safe?, Why would I use a heavily pigmented black glaze on a food surface?, Limit Recipe
Tuesday 29th June 2021