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Technical Tips Blog
Low fire mug survives two-foot drop on to cement
In slow motion. This is Plainsman Snow clay, a super white burning dolomite body. Although low-temperature ware is not as strong as stoneware it has more elasticity. Other than a tiny chip out of the base and handle, this mug still has the ring of strength. Where does this piece get all this strength? 1. It is glazed inside and out. Glazes adhered well to this porous body, forming a deep interface. The compression under which the glazes exist not only resists crazing but adds a lot of strength to the piece. Notice how many times it bounces on the handle!
Context: Fired Strength
Friday 24th June 2022
Why is dolomite body bisque is splitting after sitting around
On the loss of talc earlier this year we had to reformulate a low fire white burning body to use dolomite instead, recipe L4410P (like talc it raises thermal expansion to ensure fit of commercial glazes). As its advantages and disadvantages become evident we have been documenting them on the Snow page. A recent revelation has been the matter of rehydration of the limestone (dolomite is ground limestone): Bisque firing dehydrates it. The dolomite particles are neutralized somewhat by being isolated and having reacted to some extent with neighbouring clay and feldspar particles. Further, during dehydration, they leave considerable porosity into which they should be able to reexpand later if needed. This photo demonstrates something we have not seen in our dry climate: These 3D-printed bisque pieces have spontaneously cracked after sitting around for some time in the much damper climate of southern Ontario. In some cases, swelling occurs around the cracks. Until we can further tune the recipe to chemically tie up the dolomite take some precautions when using this type of body. Glaze ware soon after bisquing. Dry it as quickly as possible after glazing. If any surface has not been glazed then render it impervious to water penetration by using a silicone sealer. Photo courtesy of Nilou Ghaemi, Sheridan College.
Friday 24th June 2022
This crack is "spring loaded" out of the cone 04 kiln. Why?
At low temperature glazes are much more likely to craze. But it is also possible to put them under a condition called "glaze compression". This is L4410P dolomite body, formulated to have the highest thermal expansion possible without the use of talc. This inside transparent glaze is G3879C (we made this as a brushing glaze, four coats are needed on this body). That glaze was formulated to have the lowest expansion possible. As the kiln cooled the body was unable to withstand the increasing compressive forces of the solidifying glaze inside, resulting in what you see here. In the days following the firing it kept widening, branching and travelling until the mug spontaneously split in two. Another identical mug with Spectrum 700 clear inside (same outside glaze) bounced like a ball when dropped onto concrete from two feet (a subsequent ricochet right onto the handle took off a chip). But when I dropped this one the pent up stresses within exploded it into dozens of pieces.
Context: Glaze Compression
Friday 24th June 2022
Iron oxide as a fining agent to debubble a low fire transparent
These terra cotta clays were bisque fired at cone 04 and glaze fired to 04 using the 04DSDH schedule. The glaze is G1916Q, an expansion-adjustable cone 04 clear. That schedule alone is often enough to get transparent, defect free glazes in many situations. But not in this case. The solution was to add a fining agent. In this case we added 2% red iron oxide (to the top glaze). The particles of iron floating in the melt acted as a congregating points for bubbles, helping them to escape. And we got a bonus: a more interesting aesthetic. A 1% addition also worked, but not as well (we have settled on 2% iron and screening the glazes to 100 mesh). Screening out the larger particles slightly degraded the fining performance (so we have to accept the tiny specks). Iron does not always workin other situations. Other fining agents we have used at cone 6 do not work in this situation (e.g. 2% Zircopax, Alumina). Of course, this glaze will fire amber on a white body.
Thursday 23rd June 2022
Don't fire Coffee Clay past cone 6
Here is what happens at cone 8, 7, 6 and 5 (top to bottom). Don't trust a kiln sitter or controller to accurately fire to cone 6, include a self supporting cone and calibrate your controller or kiln sitter to guarantee that firings do not exceed cone 6. Make sure the self supporting cone is bending correctly. Built in firing schedules are almost certain to overfire the kiln, manually program your controller to prevent losing ware made from using Coffee Clay.
Thursday 16th June 2022
The supply chain crisis: Potters now need this
A sieve shaker. Being more independent is now cool again. Actually, it is being forced upon us by necessity because of supply chain issues and skyrocketing prices of convenience glazes, bodies, engobes, etc. Independence involves using sieves. True, it is no problem for a potter or lab tech to manually coax a glaze slurry through a small 80# sieve. But real independence is about sieving in volume - clay bodies and casting slips. About making your own porcelains and sieving out agglomerates. The ultimate in independence: Sieving particulates from your own native clay slurries. And doing it at 100, 140 and even 200 mesh. That requires a sieve shaker. This one cost us less than $100 to make. Of course, a Tyler sieve (or similar) is needed, these can be purchased on Ebay or Amazon. And a vibration motor, some metal and hardware and a friend with metal fabrication tools.
Thursday 16th June 2022
This glaze is flaking off because of the underglaze
Pure ceramic stain powders don't melt at typical pottery temperatures so they cannot bond with clay body surfaces. They don't suspend in water, even if constantly stirred a pure stain is completely hostile to being applied by brushing or dipping. Their color is far too concentrated for use pure. And they are much too expensive to even think of such. Pure stains contribute only one ceramic property: Color! Commercial underglazes sold in jars dilute stains into a recipe of materials, a 'base medium', designed to impart all the missing properties. Of course, underglaze manufacturers will want to use the same medium to make all their colors. But there is a problem with that. Blue stains often flux the medium, even at low percentages. Pink stains (also orange, lavendar and others) do the opposite, they are both refractory and must be employed in much higher percentages to achieve color, the resulting underglaze can fire so dry that it does not even bond. Clearly underglaze manufacturers should tailor the medium to the stain and target temperature. But not only do some not do that, they advertise their underglazes as suitable across wide temperature ranges! Consider this photo: Bond-failure is occurring at the underglaze-body interface of this low temperature plate and the glaze is pulling it away in large flakes.
Thursday 16th June 2022
A plaster table: Maybe better than a pugmill for you
This is an example of an angle iron utility table being made into a plaster table. The cardboard sides extend upward to make the slab thicker and create a buffer gap to prevent the expanding and setting plaster from pressing outward on the frame. 150 lbs plaster (92 lbs water) was poured into the plastic-lined space (the bottom cardboard sections were supported from below). In a dry enough climate, this table could make enough clay to support slurry-to-plastic production for a potter (a thicker slab would enable even more capacity). The slurry up process is better than a pugmill for small operations. It's much cheaper and is an easier way to utilize scrap material and weigh out custom recipes. The clay quality and de-airing is better (without hard chunks and contamination common to pugmilling). The procedure generates much less dust and the tank is easily cleaned. Slurries are easily sieved if you have a sieve shaker.
Tuesday 14th June 2022
An underglaze tissue transfer with clear overglaze at cone 6
This was applied at leather hard stage on Plainsman M370, bisque fired on, dipped in clear G2926B glaze, then fired at cone 6. The transfer was purchasing online. Since the pigment contains cobalt it does feather somewhat at the edges, this would be less of an issue at low temperature.
Wednesday 8th June 2022
How to make this glass pooling effect in pottery bowls
These are called "Little Dishes" and are sold in tourist areas. They are made in the USA. They appear to be fired at cone 10R (because of the appearance of the glazes and bare clay on the foot). After glaze application and pigment banding, a thick layer of glass powder (glass cullet) is poured in. Since this type of glass has a high CTE it crazes thoroughly during cooling. Of course, that weakens the piece, but since these are decorative the aesthetics are considered more important. This effect can be achieved at any temperature by just using frit powder, Ferro frit 3110 will melt and craze like this at cone 04 or even lower. Suspended bubbles will be a problem, use a firing schedule with a hold to give them time to surface. Also, use a body having low LOI so that it is not generating gas bubbles. Bisque firing pieces at a higher temperature will accomplish the same (for example, if you are making these at cone 04 then bisque fire at cone 03).
Tuesday 7th June 2022
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