Mason Stains in ^6 Glazes

In my second round of testing, I was optimistic that I’d found the holy grail of base glaze recipes to achieve a pink ^6 glaze – but I was mentally prepared to open the kiln this morning and find nada.  Imagine my surprise and delight, when I opened my kiln and found these gorgeous tiles!  After loads of research and internet sleuthing, I happened upon this article by Tony Hanson that included a recipe for a base glaze good for chrome/tin colorants on the ceramic-materials.com on Sunday morning.  I quickly made up 3 test batches of the base glaze and fired up my little test kiln yesteray with anticipation.

To be expected, color developed much better after using a higher percentage of colorant.  I decided to try up to 12% stain this time just to compare with my tests a couple of days ago where I used up to 10% stain.  Truth be told, 10% is plenty – but I wanted to see if it made any difference.  It looks like color doesn’t start to really develop until tile 3 which would be 25% of the total amount of stain used (in this case, 25% of 12 = 3% of the total 100 gram recipe which results in a very pastel color).  I’m guessing that when I test some blues & greens, I will need even less colorant since cobalt and copper are much stronger colorants.  Also, I’m going to try redoing the pink glaze using chrome oxide and tin with the new base glaze.   Yesterday, Annie generously forwarded another chrome/tin compatible recipe to try and my friend Mary Cay emailed me this morning reminding me about a recipe that she included in the materials from a glaze workshop I took with her last year.  Thank you to both of you!

I also retested the chartreuse stain using the pink base glaze #4 (used with the pink and violet stains sucessfully) from Tony Hanson.  I’m interested in have 1 or 2 base glazes so that I only have to buy minimal ingredients in bulk for future use.  And, I had a thought…  I plan on using a combination of 2 different glazes for my work.  For example, glaze the exterior of a cup violet and the interior chartreuse.  My thinking is that if violet is a chrome/tin glaze that can have no zinc, magnesium and needs calcium – then some funky stuff might happen where the violet and chartreuse glazes meet.  But, not if they’re the same parent base glaze (or so goes my rational).  I may be wrong and I plan on calling Mason Color later today to ask some questions.

There is a slight color difference between the 2 glazes.  The one tested in the last post is more green, whereas the one with the pink base glaze #4 truly is more mustard yellow.  If that’s what I’m going for, I could simply use rutile as the colorant at a lower cost than the mason stains.  I suppose this is the whole point of testing; to see what can be learned and how to apply the knowledge effectively.

Thermal Expansion and Clay/Glaze Fit

I still need to find out a little bit more information too, before I mix up big batches of glaze.  Kris left a comment on my last post about the thermal expansion of the clay body I use.  If the clay body and the glaze’s thermal expansion don’t jive, crazing and a host of other issues could crop up down the road.  Funny, thing is that I was just reading up on this last night in  Mastering Cone 6 Glazes, Chapter 5 “Fitting Glazes to Your Clay Body”.  I need to call Mile Hi Ceramics this morning when they open and find out what the thermal expansion is for the 2 clay bodies I generally use – Ash (^6 white stoneware) and Aspen (^6 porcelain).  If the glaze is too big for the clay body, shivering results.  If the clay is too big for the glaze, crazing results.  Shivering is not cool – chunks of glassy glaze falling off the ware.  Crazing (fine crackling) is sometimes desirable for some potters, but isn’t really a good attribute for functional ware that comes into contact with food.

Update: Neither of the people I spoke with at Mile Hi Ceramics know the thermal expansion of their clay bodies – I was told to call back when the owner is in. Boohoo.  Am I over thinking this?

Using Underglazes to ^6

I did come upon some other interesting test tiles in my studio a few weeks ago while cleaning.  These were tests that I had mis-fired last year involving Duncan Cover Coat Underglazes.  I was doing these tests to provide my kid’s clay classes reference color chips.  The kids always wanted to know what the various colors were going to look like when fired.  My mishap was due to my Aim cone 10 test kiln over firing due to a couple of very careless mistakes.  I  put a small pyrometric cone 6 in the kiln sitter, loaded the kiln, turned it on, ramped up the heat on my predetermined schedule.  My mistakes: I wasn’t around when it was supposed to turn off and when I opened the kiln in the morning I realized that the kiln had totally over fired because one of the shelves was touching the kiln sitter.  I’m not quite sure what temperature the kiln reached – the pyrometric cone was totally melted and black (normally white after a succesful ^6 firing) and the kiln fires to cone 10.  It’s a good possibility that these tests were fired to cone 8-10, depending on how long it took the pyrometric cone to melt allowing the sitter to turn the kiln off.

I used cone 6 porcelain test tiles and though they didn’t melt or even bloat, they all have a sheen to them like glaze in all the unglazed places.  So, why am I telling you this?  Because I found out that Duncan Cover Coats can be fired quite high and retain their colors.  When I get around to photographing them, I’ll post those results.  Underglazes would certainly be an option for me to use as a colorant under a clear glaze.  I should redo these tests firing them to cone 6 and compare the results.  I have everything from yellow, organge, red, violet, light and dark blues, green and black in my “failed” tests.

I originally wanted to toss these test tiles since I was angry at myself for my mistakes, but held onto them for some reason.  There’s something to learn in both our successes and more often in our mistakes.  Zen master is teaching me again

So – I’ll post those tests later this week for you.

Meanwhile, I’ll be teaching a kid’s class at the Art Student’s League of Denver starting next Monday called, “Mixed Media Marionettes”.  The class is now full and is for grades 3-5 but, lucky for me I will have an assistant to help.  I will be spending part of my time this week making the proto-type and lesson plans for the class so I’m well prepared.  I pulled my daughter’s wood marionette down from her bedroom ceiling ceiling the other day and am studying its construction so I can successfully recreate a working one next week with the kids.  I’m also deliberately working fast since we only have 5 days to work on these.  3 with wet clay and the other 2 decorating, assembling and hopefully playing with them on the last day.

Whew… have a good day,

~Cynthia