Archive for the “glaze testing” Category

Tile by Mary Cay

My good friend and former ceramics instructor, Mary Cay, popped over for a visit last Monday evening.  It was so good to see her again and she came bearing a gift for me.  She gave me the tile above that I had long coveted and was originally part of an exchange we arranged when I helped her set up her website last year.

It was so good to catch up and I was very excited to hear that she has found space to set up a ceramic studio again in the Denver area after a period of abstinence from clay due to a bit of a life change.  I’m looking forward to visiting her studio once she has it set up.  Maybe she’ll let me share a few photographs of her studio at a later date….

Bottom of mug attached to kiln shelf

I learned something valuable the other day.  I did a couple of glaze tests in my test kiln on a couple of mugs I will be swapping with other EMT members as a part of our annual EMT Secret Mug Swap this month.  Unfortunately, this one experienced a bit of a problem and one for which I’m grateful.  Why am I grateful, you ask?  Because I discovered something about using Amaco LUG Decorating Colors that will save me from making a similar mistake again.

At cone 6, the Amaco LUGs melt like a glaze - they must have some fluxing agents included in the recipe, whereas at cone 05 (the recommended firing temperature), they are a flat matte texture.  I figured that if Duncan Cover Coats can fire to up to  cone 10 and maintain their color, then the Amaco products should too in theory.

The problem is that the Amaco underglaze acted like a glaze and fused to the kiln shelf - the good news is that it only affected 1 mug and not 40 (the number I have waiting to be glazed this week).  Lesson learned.  Why does pottery have such a learning curve?

Another good discovery, the raspberry color on the exterior is left over from my cone 6 pink and violet glaze tests and is very stable - it stays put and doesn’t run AND I can brush it on which means that I could conceivably make smaller batches of the glaze so that I don’t have to make big dipping batches.

Meanwhile, I mixing glazes today since I have about 3 bisque loads to glaze in the next week and will visit the Museum of Contemporary Art in Denver later on this afternoon with my family.  Stay tuned for more on solar energy in the coming weeks.

Have a good weekend,

~Cynthia

Early this summer, I set out on a quest to expand my cone 6 glaze palette and since I’m attracted to vibrant colors, I decided to look for some candy colored hues such as pink, violet, and chartreuse.  I found success with Mason Stains as colorants in this pink base glaze #4 cone 6 glaze recipe I found on http://www.ceramic-materials.com.  After figuring out the cost per batch, I decided to try using chrome and tin oxide instead of Mason Stains to achieve a pink glaze and also to substitute some ingredients to make the cost per 10,000 gram batch less expensive.  My new criteria sent me off on another round of testing which I ended up abandoning to write some lesson plans in preparation for my teaching stints this last summer.

Cone 6 Pink chrome-tin combo

There is a slight error in this photo - there should be one more division of the glaze colorants lower right hand pile for the 6 part color progression glaze test.  In some ways, this glaze test was doomed to fail from the start because I lost the momentum of my glaze testing in July when I switched gears.

Cone 6 pink chrome tin combo ready to be fired

I don’t think I had anything to lose by proceeding with firing the test tiles even though I can’t say for certain which recipe I was using because it was not labeled - I could only make some assumptions based on my chicken scratch.  This test series also illustrates the need for taking good notes and accurate labeling in the studio.  The less than stellar results could also have something to do with the ingredients I substituted.  I still have another 2 batches of the base glaze to test, but since I’m not really 100% sure what these recipes are, I’m tempted to start over.  While I was in the heat of testing in July, I thought I would remember my reasoning and steps crystalline clear - big mistake.

Labeling my tests more accurately

So, in an attempt to be more organized in the studio - I added the date that I actually fired the tests on the reverse of each tile, along with the name of the (assumed) base glaze, numbered tiles 1-6 , clay body, temperature of clay - the latter three are scratched into the clay body before bisque firing and the former are written with a glaze pencil.

Cone 6 pink base chrome tin combo results

Results - blah.  I’m not sure if it’s the right recipe, the chemical make up of my substitutions or that my scale is off.  I had a dickens of a time measuring out .2 grams of chrome - it’s such a strong color.  After looking at the original recipe and then the “Raspberry” glaze recipe in Mastering Cone 6 Glazes, I know the proportion of chrome to tin was probably wrong.  .2 chrome - 7.5+ tin oxide would yield a decent result I think….

There is a little pink visible in the test tiles with higher amounts of tin and chrome

The latter 3 tiles are the only ones to show a hint of raspberry color - and these are the ones with the highest concentration of tin.  One interesting bit of info gleaned is that test tile #3 2nd photo up is close to the chartreuse color I’d like to use - much softer than what I achieved with Mason Stains.

Kiln is firing too hot

Another bit of information that I suspected, but have now confirmed is that two of my three of my kilns are firing hotter than cone 6.  Last time I visited Mile Hi Ceramics, I picked up some boxes of self supporting cone 5, 6 and 7 pyrometric cones.  Check out cone 7 - it’s touching the kiln shelf - the trio of cones should look like this instead.  These were placed in my test kiln which has a kiln sitter that turns off the kiln when it’s reached temp.  This past weekend I also tested my ancient Paragon - and it also over fired.  I haven’t tested my new digital Skutt 1027 yet, but will have the opportunity to do so next week sometime.  I almost have a full load to glaze.  Tips anyone?

Meanwhile, according to my studio log, I spent 12.5 hours out of my goal of 20 hours a week in the studio in the last 7 days.  I consider this a success considering that I was a substitute teacher 2 of those days and have reserved Sunday for family and getting ready for the following week.  This next week should prove more fruitful.

With that said, I’m signing off for the day,

~Cynthia

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

1. chrome 1/2% + 7% tin oxide
2. Pink stain - test successful

• Purple

1. 1/2% cobalt + 1-2% manganese
2. 3% manganese in high alkaline glazes

• Yellow Green

1. Stain - chartreuse successful, but not quite what I’m looking for
2. 5% yellow ochre + 1.1% rutile
3. .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