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Monthly Tech-Tip from Tony Hansen

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At what point is a self-supporting cone bent to the correct degree?

A self supporting cone in an Orton guide

Orton says “90 angular degrees is considered the endpoint of cone bending”. First, let's assume the normal: Examination of cones on kiln-opening to verify controller operation. Consider the cone on the left: The tip is touching. But it is also beginning to buckle, which means it was touching for a while before the firing ended. Who knows how long! The second one is not touching but has still fallen a little too far. Why do we say that? The third one, positioned on the Orton guide, has reached the recommended 90 degrees. This demonstrates a good reason why self-supporting cones are much better than standard ones: They are not touching when considered done. And standard cones, when sent in a 3/4" plaque, have a less consistent bending behaviour.

Context: The bending of an.., Cones bending badly, Are you using your.., Manually programming a Bartlett.., What temperature do Orton.., Program your firings manually.., Cones bending theoretically cones.., Pyrometric Cone, Make Your Own Pyrometric..

Saturday 20th April 2024

This iron oxide stain on porcelain disappeared during firing?

Iron stain porcelain fires clean

This porcelain is 43% kaolin, 20% silica, 36% nepheline syenite and 1% Veegum. I cast it in a test mold that had been used for a very dark red burning body. The inner unstained section looks no different than the outside after firing to cone 6. This porcelain develops a vitreous surface, it is apparently able to dissolve and absorb the thin film of iron (unlike a stoneware). Kaolins in even very white burning porcelains always contain a small percentage of iron oxide as a contaminant, but as long as it is below about ~0.5% the fired color is not visibly darkened.

Context: This is how much..

Wednesday 17th April 2024

First mug in my newly created mold

Slip cast mug

This test mold is thin-walled yet I can cast three thick-walled mugs in three hours. This clay is L2596G, a buff burning cone 10 stoneware - the mug on the lower right has been fired to cone 10 oxidation. Achieving 4-5mm thick walls is not a problem if the casting slip employs a large particle kaolin intended for this purpose (e.g. Opticast). The flared lip works as expected, keeping the rim nice and round. No cracks have appeared at handle joins, even for pieces left in the mold overnight. The mold halves mate with each other very well and the seam is easy to remove. The seam on the base is an issue - I have to be careful to line up the halves well before clamping the mold strap - this is a warning for accuracy during the mold production stage. And the possible motive for a three-piece mold.

Context: AI-imagined mug I chose.., Coffee Mug Slip Casting..

Wednesday 10th April 2024

Personal size sieve shakers you can buy on Amazon

Small vibrating sieve shakers

These are small-scale devices that work on the same principles as industrial ones - thus they put industrial methods of clay processing in the hands of a potter or hobbyist. These products are stainless steel and food grade and most can be used with powders or shurries and have interchangeable screens. The most expensive device here costs $5000 and the least expensive is far less than $1000. The suppliers of each of these also have other similar machines (as well as other types of processing equipment potentially valuable in small-scale ceramic production).

Context: How to Find and.., Formulating a body using.., 11 v 22 v.., 11 v 5 w.., 22 V 5 W.., 2 8V 6 HZ.., Making your own sieve..

Sunday 24th March 2024

3D printed plaster filled case mold ready for pouring block mold

3D printed mug mold

This is part of a project to make a slip-casting mold for a coffee mug. In the slicer, I split the print into two pieces 22mm up from the base. This enabled doing the bottom section right side up and the top one upside down. That drastically cut the amount of support generated (and thus printing time). I scotch-taped the two halves together and filled it with plaster to produce a rigid block mold. The two halves fit so precisely it is difficult to tell where they join. The big benefit of printing it upright like this is that the all-important front face is very flat (there is some warpage on other parts but that does not matter).

Context: Coffee Mug Slip Casting..

Wednesday 20th March 2024

3D design, printing and use of a slip clay test bar mold

SHAB mold for casting clays

This is for making test bars of slip casting clays bodies for use in the SHAB test (to measure drying shrinkage, firing shrinkage and fired porosity). I designed it in Fusion 360 and 3D printed the light-duty rails and case mold. I poured plaster into that to make the two plaster working mold halves (top right). The funnels provide a reservoir so the bars be cast solid. This mold can produce a set of three bars in less than an hour.

Context: Shrinkage/Absorption Test, Side Rails

Tuesday 19th March 2024

Lead bisilicate with his ugly borosilicate cousins at a cone 05 party

Boron and lead transparent glazes on terra cotta

The middle front mug is glazed with an 85:15 lead bisilicate:kaolin mix, the G3971 recipe. It is an absolutely "knock your socks off" crystal-clear hyper-glossy surface that transmits the terra cotta color beautifully regardless of whether the clay is smooth or coarse or the glaze thick or thin (this one was applied as a brushing glaze in three coats on L215). My lead testing kit passes it with no detectable lead release. The other pieces are done using brush-on versions of boron-based clear glazes (commercial and made from a recipe). At almost any thickness and whether on L215 or L4170B clouding occurs. The worst one is a commercial three-coater on the right, the best is G1916W (it has 2% added iron as a fining agent for the micro-bubbles). My terra cotta plan: Glaze the inside functional surfaces with that and the outsides with the leaded one (and using a kiln exhaust system).

Context: Lead Bisilicate Frit, A lead bisilicate frit.., 2 iron oxide in.., Boron blue in low.., A cure for long-time.., Is it impossible to.., Thick application clouds a.., Lead in Ceramic Glazes..

Sunday 17th March 2024

A better cone 6 oatmeal glaze using Ravenscrag Slip

Ravenscrag oatmeal glazed mug

Left: G3933EF oatmeal based on Ravenscrag Slip.
Right: G3933 oatmeal based on a mix of G2934 matte and G2926B glossy base glazes.
Both have the same added colorants. The Ravenscrag version features several advantages. Most importantly much less tendency to crawl. It has better application properties, the slurry needs less water and it is naturally thixotropic. It has an extra option for adjusting properties: Changing the ratio of roast-to-raw Ravenscrag clay. It is responsive to cooling differences - more matte on slow cool versions of the C6DHSC schedule (e.g. 150F/hr), more glossy on faster cools (e.g. 250F/hr). And, its recipe is adjustable (e.g. raising the MgO if a more persistent matte is needed). And, it looks and feels way better, interacting with dark bodies for richer color and varying in tone more for thinner and thicker sections.

Context: Ravenscrag Slip, Sometimes it is better..

Wednesday 13th March 2024

Use the same runny glaze as its own catch glaze

As runny glaze as its own catcher

This is G3948A, a super runny cone 6 iron red glaze. The clay body is M340. This glaze has to be runny, applied thickly enough, be held at temperature and cooled slowly to achieve this visual effect. When applied at the needed thickness it will run off the ware onto the kiln shelf during firing. Why has that not happened? A catcher glaze on the lower section. In this case, the catcher is the same glaze. On the left, the bottom half of the mug has just been dipped into the glaze quickly, giving a layer that is too thin to achieve the red effect. That dried within a few seconds and enabled pushing the top half down into the dipping glaze for twice as long (the inside has a liner glaze and is waxed up to the rim). The upper section glaze is guaranteed to run and the bottom is not thick enough to run. The result is complete blurring of the dividing line and coverage that looks natural and flawless.

Context: Stop a runny glaze.., Catch Glaze

Tuesday 5th March 2024

ChatGPT woke me up about making my own frits.

Frit melted in crucibles

I asked ChatGPT this question and got a very thoughtful answer that seems to confirm what I have observed in smelting material mixtures into ingots in alumina and zircon lined slip cast crucibles. The mixtures are melting well in the test crucibles (the upper one was fired at cone 4, the lower one at cone 6). But there are issues. There does appear to be phase separations. And bubble froth at the top. For some compositions alumina works better as a liner, for others zircon. We are getting closer to trying larger multi-kilogram batches and ball milling so time will tell.

Context: Crucible with alumina oxide.., AI in Ceramics

Friday 1st March 2024

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