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.
Context: Interpreting Orton Cones, Cones, Plainsman Cone 6 Slow Cool (Reactive glazes), Cone 6 Drop-and-Soak Firing Schedule
Wednesday 20th 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.
Context: G2934Y - Cone 6 Magnesia Matte Low LOI Version
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.
Context: Ceramic Tile
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.
Context: G2934Y - Cone 6 Magnesia Matte Low LOI Version, Crawling, Glaze Crawling
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.
Context: Deflocculants: A Detailed Overview, Understanding the Deflocculation Process in Slip Casting, Deflocculation, Slip Casting
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.
Context: Wood Ash Glaze, Lime Popping
Wednesday 23rd October 2019
Why throw on a plaster bat when making larger pieces?
Even drying, that's why. As soon as was practical I covered the piece with a cloth and then put it inside a garbage bag. While that put the upper section a little ahead in drying, over night the base caught up as the plaster sucked the water out. In the mourning I lifted it off without having to cut it, after an hour or so it was stiff enough to trim. I secure the plaster bat to the wheelhead using a "Batmate", that works extremely well. A thin layer of slip applied to the batt and firmly throwing the piece of clay down onto it sticks it well (it requires practice to hit center). No other method will assure even drying throughout and prevent cracking as well as this.
Context: The Black Art of Drying Ceramics Without Cracks
Tuesday 22nd October 2019
Getting a 31 inch porcelain plate through drying and firing without cracks
What does it take? Three months! Porcelains are fine grained and, for heavy pieces, they will not tolerate uneven drying at any stage. These cone 10 plates are made by Peter Flanagan at Okanagan Pottery in Nelson, B.C. Firing is also a real challenge. Pottery porcelains are high in quartz, getting a piece like this down through quartz inversion
(~1200-900F) without dunting is only possible if done very slowly. The fact that ancient Chinese potters made very large porcelain pieces means they knew about slow cooling also (and it was a natural consequence of the heavy kilns they used). But our modern kilns cool quickly so the drop must be slowed. Peter adds an extra level of "super humanness" by actually lustre firing these pieces, that means 2 more trips through the hazardous quartz inversion
territory! If you do this be prepared to fire super slow (e.g. 25 degrees per hour) through this range.
Context: The Black Art of Drying Ceramics Without Cracks, Quartz Inversion
Tuesday 15th October 2019
For even coverage white majolica glazes must be applied by dipping
The mug on the left has three coats of Spectrum Majolica base, painted on by brush. Drying was required after doing the inside coats, so the total glazing time was several hours. The glaze layer is way too thin and it is not even at all! The one on the right was dipped in a 5 gallon bucket-full of Arbuckle white (that was weighed out according to a recipe and slurried at 1.62 specific gravity
). It took seconds to dip-apply, the thickness coverage are good. As is obvious, it makes sense to make your own base white. Then, you can decorate using the overglaze colors (e.g. the Spectrum Majolica series). Another advantage of making your own white is that you can splurge on the amount of opacifier
(in this case 9% zircon and 4% tin oxide), to achieve maximum whiteness and opacity. And, you can proportion a mix of two frits
(having higher and lower thermal expansion
) to fine-tune the fit with the body (a big issue at low fire).
Context: White majolica bases have very, very low melt fluidity, Majolica, Brushing Glazes, Base-Coat Dipping Glazes
Monday 14th October 2019
Does a grog addition improve thermal shock resistance?
Pyrax (Pyrophillite) is a mineral having a very low thermal expansion
. It stands to reason that if we can maximize its percentage in a body and not fire the body to a point that changes the crystal structure, it will be resistant to thermal-shock-resistant cracking. To that end I mixed it with only kaolin (ball clay would add some quartz that would increase thermal expansion
) and made slip-cast pieces. I fired them to cone 2 (after finding that by cone 4 shock-resistant properties begin to decline). As you can see from the video, the addition of grog actually harms the performance! The higher the Pyrax, the better. Will this work for kiln shelves? Yes!
Context: Pyrophyllite, Thermal shock
Friday 4th October 2019
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