Duncan Cover Coat Results Fired Between Cone 7 and 10 in Oxidation
Warning - yawner of a post unless you’re interesting in ^6 glaze testing!
Recap: looking for candy colored transparent cone 6 glazes
As promised in my last post, here are the results of over firing Duncan Cover Coat Opaque Underglazes for use on greenware, soft fire bisque or bisque in an electric kiln between cone 7 (2264°F or 1240°C) and 10 (2381°F or 1305°C). The reason I can’t tell you exactly how hot the kiln was fired is because I made a couple of careless mistakes; a shelf was touching the kiln sitter and I was not present when the kiln shut off - basically the kiln shut itself off after the pyrometric cone in the sitter melted down, reached cone 10 or when the timer went off. I now keep a good kiln log and know that it takes my test kiln approximately 9 hours to reach ^6 on a slow fire with 2 hours on low to start. I now set the timer for about 9:30 minutes to prevent the kiln from overfiring and I double check the pyrometric kilns for the sitter to make sure I’m using a ^6 vs. a ^06 or ^04 one.
Duncan recommends painting of 3 coats of underglaze for full coverage and if using for food, to brush on a clear glaze and to fire to shelf cone 06. For you non-clay people cone 06 (1830° F or 999° C) is low fired ware usually earthenware. I normally fire to cone 6 in an electric kiln or to 2232°F (1222° C).
I didn’t keep good records when I made these tests. I was merely trying to fire some color test tiles for my kid’s clay class to use as reference when decorating their work. I also broke a rule, I used my ^6 porcelain clay body since I had the test tiles already made up, instead of the low fire white earthenware clay that they used. But, I figured that they’d get a good idea of what the color would look like when they saw the tiles. So, I’m not sure if I was going to fire these to cone 06 or cone 6. Good records would help me interpret these tests now! Live and learn.
So, here’s what I do remember doing, I painted 2 coats of the underglaze on the tiles, then I only glazed the right half of the tiles with a clear glaze. It was either a commercial low fire clear or a very basic good ^6 clear glaze comprised of 50% gerstley borate, 20% kaolin and 30% silica. I see this recipe all over the place. It turns milky where thick (on account of the GB), so brushing is best.
So, I just input the 3part clear glaze recipe into Insight, the glaze calculation program I’m testing, and found out something interesting. The clear glaze contains .17 MgO which tells me that it is not compatible with chrome/tin colors so I most likely used the cone 06 commercial glaze for these tests since the violet didn’t burn out. I’m still guessing though. It does explain why the pink on my cherry blossom porcelain box burned out the last month. 
The formulas are, however, proprietary and a trade secret, so I’m making some assumptions about Duncan’s product. I can only make general guesstimates as to the coloring oxides that they use - I would think that they’re similar to Mason Stains.
I can tell you that with CC#103 Lotus Yellow, the color burned out between ^7 - ^10. CC# 105, Sungold Yellow looks like a tan yellow or straw color. CC# 142, canary yellow is bright lemon yellow and CC#185, Apricot is a bright orange. What’s interesting is that the first 3 test tiles are glossy on the left hand side where I didn’t apply the clear glaze which means that the additional frits or whatever else was used makes these self glazing at higher temps. The Apricot is matte where unglazed.
CC #186 Tomato Red is a bright red and has a nice sheen on the unglazed portion of the tile. CC#189 Violet is a light lavender color - the unglazed portion is matte. CC#134 Stardust Blue is a pale baby blue color - matte where unglazed. CC # 159 Bright Blue is a royal blue and matte where unglazed.
Velvet U/G #343 Chartreuse (and my only exception to the Duncan test) is a light chartreuse color, matte where unglazed. CC #158 Bright Green is a dark olive green at higher temps and is self glazing. CC#154 Cobalt Blue is a matte metallic black where unglazed and a midnight blue/black where glazed. CC#165 Black Brown is black on both tiles, however on the unglazed portion it bubbled and has pinholes. On the glazed side, it’s glossy black.
I search for a lot of ceramic information on the web and find it incredibly helpful when photographs are included. For example, I would love to see photos with these tests done with Ducan Cover Coats to cone 10 in oxidation and reduction. The clear glaze used on top of these tests is:
whiting 23.5 silica 34.0 EPK 18.0 Custer Feldspar 24.5
What’s interesting is that this clear also contains no MgO - so it will also work with chrome/tin combos as well.
So, now what? After figuring out the cost of the pink and violet glazes using Mason Stains, I’m going to see if I can come close using oxides as colorants to reduce the cost.
For a 10,000 gr batch of the Pink Base Glaze #4 used in my last post, provided that I purchase my ingredients in bulk would be $20.52. If I use the pink mason stain as a colorant it is $16.85/pound right now at Mile Hi Ceramics. At 8% for a nice pink color that would mean I would need 800 grams of stain in a 10,000 gr batch which would mean that I would need 1.76 pounds of stain at a cost of $29.69/ 10,000 gram batch. Ouch.
If I can recreate something similar using chrome and tin at around 1/2% chrome oxide and 7% tin oxide, I would need 50 grams of chrome and 700 grams of tin for a 10,000 gram batch at a cost of .90 ¢ and $26.36 respectively for a total of $27.26.
Okay, so there went that theory! It’s the tin that’s the expensive ingredient. So, I guess if I want a pink or a violet, I should just use the stains for convenience since they cost the same. Or, I might decide to use an underglaze to paint on the color and cover with a clear since that worked equally as well based on the test tiles above.
Colorants I plan on testing:
- Pink
- chrome 1/2% + 7% tin oxide
- Pink stain - test successful
- Purple
- 1/2% cobalt + 1-2% manganese
- 3% manganese in high alkaline glazes
- Yellow Green
- Stain - chartreuse successful, but not quite what I’m looking for
- 5% yellow ochre + 1.1% rutile
- .25% - .5% chrome oxide in high alkaline glaze
I know I can get a great light blue, turquoise, green, black and white. But, for some of the other harder to achieve colors, It might end up being more economical to use the stains or underglazes.
And, here I thought these recently rediscovered test tiles were failed tests!
Must start making some more test tiles…
~Cynthia
