I’m not a plaster master, so I don’t know if this is a good or even original idea. By mixing plaster in a plastic bag, it seems easier to remove bubbles from the mixture, while pouring is much more controlled. It’s the same idea as using a garden watering can for pouring glaze: You pour from the bottom where pressure is highest and bubbles are fewest.
In Chinese Glazes, we learn from Nigel Woods that the cobalt used for underglaze blue & white underglazes and blue glazes came in a range of chemical compositions and grades of purity. Thus, there are many shades of blue due to the quality of cobalt-containing stone as well the overlying glaze.
In the same book, Nigel presents a lovely Chinese blue stoneware glaze which, in addition to cobalt, contains iron and manganese “impurities”.
In fact I’m personally not fond at all of glazes and underglazes containing only cobalt as a coloring oxide. Pure cobalt often comes out as a garishly blue color. In the triaxial blend below, I take a nice clear glaze (Sue’s Clear) with added 1% Cobalt Carbonate. Then I blend with 1.5% Red Iron Oxide (bottom left) and 1.5% Manganese Dioxide (bottom right). The resulting colors on the bottom row are much more pleasing to my eye.
The full image can be viewed here: http://www.derekau.net/wp-content/uploads/2014/12/BLUE_TRIAX_ALL.jpg
Having not fired cone 6 since college, I started by first testing a number of clear cone 6 glazes on https://glazy.org
Out of these tests, there were two glazes that I preferred. The first, Sue McLeod’s Clear, is a soft clear with minimal clouding and has B2O3 at 0.18 which, according to Matt’s information, is ideal for cone 6 glazes.
The second glaze is a shop glaze available at the Wellsville Creative Arts Center called WCAC Celadon Clear. With B2O3 at 0.45, it is really high in boron and possibly less durable than the lower-boron clears I tested. However, WCAC Celadon Clear is by far the clearest glaze I’ve tested, almost like a layer of pure glass or honey. Even on dark stoneware it’s really clear with almost no clouding.
Being new to cone 6, I was curious as to the effect of boron levels on clear glazes. So, I created two biaxials, both with R2O fixed at 0.2. In the first Sue’s Clear inspired biaxial, B2O3 is set at 0.18. In the second biaxial inspired by Celadon Clear, B2O3 is doubled to 0.36.
Each biaxial resulted in a nice clear, with the higher Boron clear being almost completely transparent and glossy, while the Boron 0.18 clear is translucent and soft.
Standard Cone 6 Porcelain Body #551
Same chart but with words describing each test glaze:
Best Clear at B2O3 0.18
The best clear resulting from the B2O3 0.18 biaxial is here: C6 R2O 0.2 B2O3 0.18 Best Clear
B2O3 0.36 Biaxial
In order to test the effect of higher B2O3 levels, I doubled the amount of Boron in the initial biaxial from 0.18 to 0.36 while maintaining the same R2O:RO ratio. I also made the boundaries of the tests a little higher (see map comparison). I was surprised to see that the only clear glazes in the 0.36 Boron test appear much farther down (lower in Si & Al) in the chart. But the “clear” region is still in the same Si:Al Stull region.
After testing WCAC Celadon Clear and seeing the results of my B2O3 0.36 biaxial, it seems there is definitely a region of very glossy, very clear glazes at higher boron levels.
Coincidentally, I tested an old glaze recipe posted to the Clayart mailing list by Laura Speirs in 1996: https://glazy.org/recipes/21102 As with the WCAC Celadon Clear, the Speirs recipe is also very high in Boron (0.51), and it also fires very clear and glossy:
VC Easy Glossy
One afternoon I began discussing the WCAC Celadon Clear with a WCAC member, Nancy Alt. I was very surprised to discover the interesting history of this glaze.
In 2009 Nancy Alt had visited Val Cushing’s home and purchased a vase with a lovely blue-green celadon glaze. Nancy asked Val if he could share the glaze recipe, and he not only shared it but converted it from cone 9 to cone 6 (the temperature Nancy was firing). Val’s email is copied below. It shows the extremely generous nature of this amazing potter and teacher:
From: Val Cushing
Subject: Re: celedon glaze
Date: May 12, 2009 at 12:46:57 PM EDT
To: Nancy Alt
This glaze is one I made for C/9 oxidation electric firing, so that it would appear to be a blue green celadon. I have revised it for you to be the same color and texture only for C/6 ox. electric . I will give you two to try , first VC Pale Emerald, C/6 , glossy , blue/green , celadon looking. as follows………. Kona F/4 feldspar 24, Ferro Frit 3134 24, Dolomite 4, whiting 14, barium carbonate 2, zinc oxide 2, flint 24, and EPK 6. ADD TO THAT , 1/2 % COPPER CARBONATE for blue green. VC/easy glossy, C/6 ox. , electric , celadon looking , green. Cornwall Stone 46, Gerstley Borate 20, Ferro Frit 3124 26, Ball Clay 8. — add 2% copper carb. and 1/2 % red iron oxide for celadon looking green color. Test these two Nancy and if the color is not exactly what you expected let me know and we can make a revision. We may have different “tastes” about color , but we can get what you want…My pale emerald should be quite a bit like the glaze on the jar of mine you now have. and THANK YOU . Val
So it turns out that the glaze I liked so much, WCAC Celadon Clear, was actually a Val Cushing recipe called “Easy Glossy”. I checked Cushing’s Handbook for the recipe and didn’t find it. Nor could I find similar recipes in the Glazy database. So it’s quite possible this is a newly discovered Val Cushing glaze recipe.
However, the WCAC Celadon Clear had been modified from the original “Easy Glossy”, most notably subbing Gerstley Borate for Gillespie Borate. I wanted to see not only the original recipe but also the color variations that Cushing was working with. So I created a triaxial blend.
Below is the triaxial blend using Copper Carbonate and Red Iron Oxide.
Some of Robert Tichane’s glaze tests and reproductions of Chinese Glazes donated to the Freer and Sackler Galleries: https://
archive.asia.si.edu/ collections/edan/ default.cfm?searchTerm=tich ane&btnG.x=0&btnG.y=0&btnG =Search
For the total work required to make a single cup, it must pass through 72 hands, and only then can it become a vessel.
72 Hands is an effort to document all types of ceramics techniques. The video style is very simple- a single take of each technique focusing on the artisan’s hands. Each video is accompanied by an article with a description of the technique and photos. Videos are shot in high-resolution 4K Ultra-HD resolution which gives a clear view of the technique.
Support 72 Hands
To support the continuation of this documentary series, please consider becoming a patron.
To be updated.
Throwing large pieces on plaster bats reduces cracking issues.
Using chamois leather to attach a plaster bat to the wheel
Large sheets of chamois leather for drying cars can be purchased online very cheaply. Synthetic versions that I have tried do not work.
Recent firing with traditional porcelain stone glaze. In the past I’ve tried but failed to use modern materials like feldspar and kaolin to capture the beautiful, unctuous surface and depth of porcelain stone celadons. In this glaze the coloration is completely due to iron occurring naturally in the material.
I use X-acto blades all the time, some modified for specific tasks like carving porcelain or scraping glaze off of feet.
I’m not sure if it’s all part of a vast X-acto conspiracy, but it seems that a lot of people don’t know that these blades can be easily & quickly sharpened? While there are a number of sharpening methods (even just using bare fired porcelain) that will work, it can be tedious to get the sharpening angle right. The most convenient method I have found is an angled sharpener (pictured). Just a few quick passes through the ceramic sharpener gets the blades useable again. It’s faster for me to sharpen the blade than switch out a dull blade for a new one. (Unfortunately my sharpener is approximately 45% degree sharpening angle (> 20 degrees per side), it might be better to have a narrower-angled sharpener.)
Also note that not all X-acto blades are stainless steel. You don’t want rusty blades all over your studio or in your reclaim. A 10 or 100-pack of stainless steel #11 blades might last you a lifetime.
A step up from the X-acto blades are stainless steel surgical blades. They come in a wide variety of shapes and sixes perfect for a number of jobs. I usually use the blades without a handle. They come in ten-packs and last a really long time if you sharpen them.
I’m sure that using a garden watering can for pouring glazes is a common technique, but when I came up with the idea I thought I was a genius 🙂 The design of a watering can ensures a constant, strong stream of liquid during pouring that is perfect for glazing. Bubbles are reduced since the watering can pours liquid from the bottom of the can.
Pouring the Outside
Once the inside is glazed, I will wait until the next day to glaze the outsides. It’s important not to overload the bisque ware with water.
I use an old electric wheel for pouring the outsides. It’s important to rotate the wheel at sufficient speed so that glaze does not gather on the inside rim of the pot.
Well, that didn’t work out. The kiln master ended up over-firing, past Chinese cone 10. Orton cone 12 probably dropped around Chinese cone 8/9. Looking forward to doing a better test in my own kiln.
I’ve finally gotten a new low-fire electric kiln. This kiln is designed to fire up to 1000°C, so it’s useful only for on-glaze enamels and bisque. Total cost was 2900RMB, which is about $420USD.
I have a couple “wet boxes”. These are plastic bins with lids into which a layer of plaster has been poured. The plaster is kept wet in order to maintain humidty, slowing (if not stopping) the drying process.
However, I haven’t used the wet boxes in a long time. I’ve found it much easier and more convenient to simply wrap each piece in it’s own plastic trash bag. Pieces stored in this manner can be trimmed weeks or even months later.
This page is in progress and will cover my kiln and firing. For now it is just a place to store my notes.