Amaco PC-20 vs Ravenscrag Floating blues

They have advantages over the original recipe

G2917, which I mixed as a brushing glaze, is on the right. This is not sold in jars, I make my own labels to demonstrate that DIY brushing glazes are not only possible, but can be better. I make the labels by ink-jetting onto regular paper, cutting them 62mm wide (2 7/16") and holding them securely in place using 2 7/8" transparent packing tape. The G2917 glaze is less melt-runny than typical floating blues. It contains the standard rutile/iron addition and exhibits good variation between thick and thinner applications. It also adds a little cobalt to ensure a good blue color no matter how the kiln cools. When I want more of the floating white colouration, I use the L4655 Alberta Slip floating blue recipe (left). Strangely, it contains no rutile, iron or even cobalt - it employs titanium dioxide (using iron available in the body and slow cooling as the mechanism of the blue). Both recipes have a big advantage over the original G2826R recipe from 50 years ago: they don't contain any Gerstley Borate, so the slurries don't turn to jelly in the bucket, this creates the option of mixing them as thixotropic dipping glazes

Related Pictures

Titanium instead of rutile for floating blue

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Rutile blue glazes are actually titanium blues (because rutile mineral is an impure source of TiO2 and Fe₂O₃). The iron and titanium in the rutile react to form the floating blue effect. The GA6-C recipe has always relied on a 4% rutile addition. Its GA6-A base recipe contains significant iron (because of the 80% Alberta Slip), so could titanium oxide deliver the same floating blue effect? Yes.

These mugs are M390 clay. The top left one is the standard GA6-C (with rutile) fired using the C6DHSC slow-cool firing schedule (the bottom left normal cool PLC6DS schedule produces little color). But the ones on the right switch the 4% rutile for titanium dioxide (the L4655 recipe). The top right was fired using the slow cool, the bottom right was the normal cool schedule.

Titanium thus seems more consistent and reliable material than rutile. An excellent blue color is produced even without a slow cool (lower right).

Here is my setup to make brushing glazes and underglazes by-the-jar

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Although I promote DIY dipping glazes, you can also make DIY brushing glazes. Let's make a low SG version of G2934BL. Weigh out a 340g batch of dipping glaze powder. Include 5g Veegum (to gel the slurry to enable more than normal water) and 5g CMC gum (to slow drying and impart brushing properties). Measure 440g of water initially (adjusting later if needed). Shake-mix all the powder in a plastic bag. Pour it into the water, which is blender mixing on low speed, and finish with 20 seconds on high speed. This just fills a 500ml jar. In subsequent batches, I adjust the Veegum for more or less gel, the CMC for slower or faster drying and the water amount for thicker or thinner painted layers. Later I also assess whether the CMC gum is being degraded by microbial attack - often evident if the slurry thins and loses its gel. Dipping glaze recipes can and do respond differently to the gums. Those having little clay content work well (e.g. reactive and crystalline glazes). If bentonite is present it is often best to leave it out. Recipes having high percentages of ball clay or kaolin might work best with less Veegum. Keeping good notes (with pictures) is essential to reach the objective here: Good brushing properties. We always use code-numbering (in our group account at Insight-live.com) and write those on the jars and test pieces. This is so worthwhile doing that I make quality custom labels for each jar!

Videos

Links

Typecodes	Floating Blue A popular cone 6 glaze that employed Gerstley Borate, it was very troublesome to use and to fire. Much work has been done to create alternative recipes.
Glossary	Brushing Glaze Hobbyists and increasing numbers of potters use commercial paint-on glazes. It's convenient, there are lots of visual effects. There are also issues compared to dipping glazes. You can also make your own.

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