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Technical Tips: Next 10

Deep, deep blue without any cobalt. How?

These have to be seen to be believed, it is the deepest, richest blue we have ever produced. This is Plainsman M340 fired to cone 6. Black-firing L3954B engobe (having 10% Burnt (not raw) Umber instead of the normal 10% Zircopax) was applied inside and partway down the outsides (at the stiff leather hard stage). The incising was done after the engobe dried enough to be able to handle the piece. The glaze is Alberta Slip rutile blue. Firing schedule: Cone 6 drop-and-soak.

Wednesday 20th November 2019

The matteness of this glaze depends on the cooling rate

This is the G2934Y matte cone 6 recipe with a red stain (Mason 6021). The one on the left was fired using the C6DHSC slow-cool schedule. The one on the right was fired using the drop-and-soak PLC6DS schedule. The only difference in the two schedules is what happens after 2100F on the way down (the slow-cool drops at 150F/hr and the other free-falls). For this glaze, the fast cool is much better, producing a silky pleasant surface rather than a dry matte.

Wednesday 20th November 2019

What position should the cone be for correct firing?

Four degrees F. These are self-supporting cones, use these. I was consistently getting the cone on the left using a custom-programmed firing schedule to 2204F. However Orton recommends that the tip of the self supporting cone should be even with the top of the base, not the bottom. So I changed the temperature to 2200F and got the cone on the right. But don't assume your kiln fires cone 6 at 2200F, it could be much higher or lower, depending on your pyrometer.

Wednesday 20th November 2019

A gunmetal glaze I have wanted my whole life!

After 40+ years of making pottery I finally have a perfect gunmetal black. It has an incredible silky glaze. It does not cutlery mark. It does not craze on anything. It is easy to clean. This is G2934Y with 6% Mason 6600 black stain. I had to tune it a bit, adding about 10% glossy G2926B, because it was a little too matte on the first firing. But now its perfect. These are heavy mugs made using the M340 casting recipe (and the casting jiggering process). The speckled mug was made by casting a thin layer of the speckled version of the slip first, then filling the mold with the regular slip. I used a 40-minute cast to get walls nice and thick!

Tuesday 19th November 2019

Hand-tooled jigger model vs. 3D-printed and cast

I am creating molds for a casting jiggering process to make mugs. I have a profile drawing I want to match (upper left). The solid model on the left is my first attempt at manual tooling. The metal template was time-consuming to make by hand, it worked poorly (the surface is rough). And the contour matches the drawing poorly. I lost the enthusiasm to even get it smooth. For the one on the right I 3D-printed a shell, poured the plaster in and then smoothed it off a bit on the wheel after the set. It matches the template and it is perfectly round (because I have a good 3D printer, a Prusa MK3S). This is revolutionary! That drawing: I hired someone on Upwork.com to draw it for me using Fusion 360. He draws things in such a way that I can fine-tune them. Then I print them using my own 3D slicer.

Saturday 16th November 2019

G2934Y glaze on Standard #112 body at cone 6

Produces an appearance very similar to dolomite-matte-glazed ware fired in cone 10 reduction. The effect would be similar using speckled bodies made by other manufacturers. Pieces made by Tom Friedman.

Thursday 7th November 2019

Is it possible to make a thin flat porcelain tile from a plastic pottery body?

Yes. The body is Plainsman M370 (~ 25 silica, 25 feldspar, 30 kaolin, 20 ball clay + talc to tune maturity). It is 3.8 mm thick fired (vs. commercial tiles at 5-7mm). It was rolled (in the plastic state) and dried completely between sheets of plaster board. Bisque and glaze firing were on an alumina shelf in an electric pottery kiln (at 300F/hr up through quartz inversion on the glaze firing). Cooling on both firings was free-fall in a fairly empty kiln. It is flat and flexible enough that I could lay it on the cement floor and stand on it without it breaking! Of course, to produce these consistently special furniture that sinks minimal heat and a kiln that can evenly apply it front and back are needed. This is doable for custom applications. Of course, to compete in the commercial market, they need to be dust-pressed and there are lots of specifications to meet.

Sunday 27th October 2019

Same body, same glaze, same firing. Why did one crawl?

The body: M370. Glaze: G2934Y (with added green stain). Firing: Cone 6 drop-and-hold. Glazing method: dipping (using tongs). Thickness: The same. The difference: Wall thickness. The one on the right was cast thinner so the glaze took a lot longer to dry (the bisque lacked sufficient absorbency). Common pottery glazes contain clays which need to shrink somewhat during drying. The bond with the bisque, although fragile, is normally enough to prevent cracking during drying if: drying occurs quickly. That happens when the body has enough porosity to absorb all the water quickly. Otherwise, cracks appear and these become crawls during firing. A complicating factor is that stain and/or zircon additions make an already-crawl-susceptible glaze even worse. Solution: Heat bisque before dipping, glaze the inside and outside separately (with drying between) or increase calcined kaolin:raw kaolin ratio in the glaze.

Sunday 27th October 2019

By the magic of delflocculation, this powder will mix into that water and still fit in the container

This is 1100cc of water and 3000 grams of M370-2 casting. Amazingly, it is possible to get all that powder into that little bit of water. And still fit in the container (2250cc) and still produce a very fluid slurry for casting. How is this possible? That water has 11 grams of Darvan 7 deflocculant in it, it causes the clay particles to repel each other such that you can make a liquid with only a little more water than is in a throwing clay! This is a test mix of M370-2 casting (it uses a large-particle kaolin), my pieces cast in 7 minutes (less than half the normal time). Using a good propeller mixer (in a bigger container of course) the slurry can be mixed silky smooth in a couple of minutes.

Wednesday 23rd October 2019

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 to 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. 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. Development procedures for wood ash glazes can provide a starting point.

Wednesday 23rd October 2019

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