Returning to my regularly scheduled clay blog after going off topic the other day…

I really want to expand my glaze palette and am searching for some fun translucent candy colored bright eye popping color to highlight the texture on a lot of my new hand built pieces.  I’m testing glazes in oxidation (electric kiln for you non-clay people) at cone 6 (temp 2232 degrees F) on a white stoneware clay body called Ash (available from Mile Hi Ceramics).

The glaze properties I’m looking for include:

• a tight fit (no running off my ware)
• translucency
• vibrant color
• stable – no crazing, pitting, crawling or pinholing
• food safe
• cost effective and reasonably safe raw materials

Colors I’m working on:

• Chartreuse
• Pink
• Orange
• Violet
• Robin’s Egg Blue

I already have a celadon, clear and white glazes in my repertoire.  If you’re a clay person, you probably already know that the colors I’m striving for are hard to get in the properties I’m looking for at higher temperatures outside of maybe commercial glazes (I could very well be off the mark though since I consider myself a novice glaze chemist) .  These colors are widely available in low fired earthenware pottery.  But, once you move into mid-range and high fire, many of these colors burn out mainly the pinks, reds, yellows and oranges.  Or they require a reducing (gas kiln) atmosphere in the case of reds and oranges.

To achieve color in glazes one can use oxides or manufactured stains.  There’s a few stain companies out there such as Mason, Cerdec-Degussa inclusion pigments, and Blue Heron (can’t find company website) – and maybe more that cater to commercial manufacturers of ceramics & pottery.  Oxides include copper carbonate, cobalt carbonate, iron oxides, chromium, manganese to name a few.

So, for this glaze testing cycle, I chose to test pink and chartreuse colored glazes.

Glaze Test Recipes

What you see isn’t always what you get when it comes to glazing!

I did a 6 part color progression test for each of the test glazes (one of the recipes isn’t listed as I added it at the last minute) using Ceramic Spectrum by Robin Hopper, pg. 187 as my guide.

Only 1 of my glaze tests turned out the way I expected!

The others are all duds unless you consider drab olive green pleasing… so back to the drawing board me.  I actually anticipated that 2 of them would fail after further research & AFTER I had already mixed up the test batches, but fully expected Glaze #1, 2 and 3 to turn out.  I also realized last night that I misread the glaze I really wanted to try for glazes #1 and #5 and used a different one than intended – doh.  #2 is pretty – but it’s an ineffective way to get blue when I could simply use cobalt carb.  The chartreuse stain was tested on a semi matte base glaze, which I’m not sure I like.  It’s looks more mustardy yellow than acid yellow-green.  A gloss may be better – so I’m going to keep on testing before trying a new stain.

I thought I had fully accounted for how all the different chemicals & minerals were going to react in the kiln, but I was wrong.  I became painfully aware of the fact after I downloaded 2 different trial glaze calculation programs.  I’m testing Insight (free trial for 2 months) and Glaze Master (also available as a free trial for 2 months).  I really like Insight since it felt pretty intuitive to use – but I really need to read the instruction manuals to really use both of the programs to their full advantage before making a purchasing decision.  There is also a free glaze calculation program called, appropriately, The Glaze Calculator, that I should probably investigate.  Here’s a bigger database with different glaze calculator programs that are available.

Since I’m really looking for stable glazes to use on surfaces that come into contact with food, certain chemicals are off limits and I really need to read up on glaze expansion, alumina/silica ratios and while I’m familiar with limit formulas, it’s only starting to make sense me.  I really wish I had paid more attention in chemistry class.

On one hand, I feel like I’ve got just enough information to be dangerous or simply very frustrated – and liken myself to throwing darts in the dark hoping to open the kiln and find success with any luck.  On the other hand I’ve been doing so much reading regarding glaze chemistry that I realize how much I have to learn.

Meanwhile, a fellow clay person and Etsy Mud Team Member, Eleanor pointed me in another new direction – that of lanthanides or rare earth minerals as colorants for glazes.  She pointed out these 2 articles Rare Earth Colorants by Max Campbell and Chris Keane and Exciting New Transparent Glaze Colors Using Lanthanide Colorants by David Pier.  Although, after a little bit of research, Laguna clay has stopped distributing Praseodymium which creates the greens and yellows due to steep increase in price (so this may be a dead end)- but there’s always Praseodymium yellow Mason Stain #6408 to use as a substitute.

Update: I just found this article by Tony Hanson that includes a recipe as a starting point for making a pink ^6 glaze.  My problem?  In addition to not having zinc in a chrome/tin glaze, there can be no magnesium.  If I had seen this little tid bit before (and it clearly states that in the Mason Color Chart which I neglected to see even though I’ve read the damn thing 100 times) I would have realized that my test recipes above contain Magnesium!  Doh

Back to chemistry class for me!

~Cynthia

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