PROBLEM Glaze Firing Too Dark

[Whenby]

Hello, it’s my first time posting on this forum. I‘ve only been doing pottery for 5 months. Recently, I fired a test pot I made using my Paragon test kiln and the glaze came out way too dark.

The clay I used is Standard 240 white clay (cone 5/6). I applied underglaze to a small area before bisque firing. (*note I set the kiln to come 04 but the 04 pyrometric cone did not bend). After bisque firing I applied Amaco Blue Stone (PC-22) rated for cone 5/6.

I set the kiln to Cone 6 with a 10 minute hold. The total firing time was 11 hours. I also put a cone 5 and cone 6 pyrometric cone inside. The cone 5 bent whilst the cone 6 did not.

I not sure where to start investigating the problem. Any help would be greatly appreciate!. I attached photos of the glaze I used and the final result. Thanks!

 

[Callie Beller Diesel]

Hi and welcome!

The most typical issues with glazes in the first couple of years of making pots most often involve how they’re applied. The 2 places to look first are how well the glaze was mixed before it was applied, and how thick it went on. From the information you’ve given, I don’t think your firing had anything to do with it. Your cones indicate the firing was within Amaco’s suggested parameters. With test kilns, you don’t have to go super slow on going up to replicate a firing in a larger kiln, but you should pay more attention to how fast it cools. 11 hours sounds kinda slow for something that small. You could either add another 5-10 minutes to your hold at the end, or try a drop and hold schedule. 

Unless your glaze bottle was sitting for months and you didn’t shake it well before applying, I don’t think it’s a mixing issue either. 

That leaves glaze thickness. Most often, you don’t get the colour on the company’s test tile if it goes on too thin. It could just need a third layer if you only applied 2 thick ones. The textures on the sample images appear to be heavily filled in, so that’s what I’m leaning towards first. Make yourself a small test pot with a thin layer, a layer similar to what you did on this piece, and a layer on top of that to confirm. It’s good practice to make sure you put a clay cookie/ waster slab underneath tests like this to avoid scraping shelves later. Especially if you’re ever using someone else’s kiln!

The other factor that could be at work is the underglaze you mention. If this glaze is applied over a layer of a dark or blue coloured underglaze, that could also affect the end result. 

[Min]

First 2 images (pot and test tile) really look they were in a firing that cooled slower than the pot in the 3rd image. Rate the kiln cools at can easily have that dramatic a difference in appearance. More matte and paler colour in a slow cooling kiln vs a kiln that cools quickly (like your test kiln would).

10 hours to do a ^5 glaze firing in a small kiln with no slow cool is a long time. Did you use a bisque fire setting going to ^6 by any chance? was your kiln new when you bought it? if not what do the elements look like? 

[Whenby]

Thank you all for the replies! I’m going to make some test tiles and try these suggestions.  The kiln is able to make custom schedules so I can try a faster rise and slower cool down. I’ll also experiment with thicknesses and won’t use an underglaze.

I set the kiln to cone 04 for bisque. Also, I did buy the kiln second-hand, but the original owner didn’t use it that much. It’s a Paragon Q11-A. If my next tests are still having issues, I’ll take a look at elements.

I’ll post the results when they’re done!

[Callie Beller Diesel]

I just want to say good for you for including cones to verify firing temps. They’re especially useful whenever dealing with a kiln you haven’t fired before. They’re all a bit different.

[Babs]

The glaze looks pretty thick and is running at the bottom

 Wondering If the long slow firig is giving more heatwork treatment to the glaze?.

At the top of glaze area , glaze is thinner.

[neilestrick]

The cooling rate of baby kilns is much too fast to give accurate results compared to a full size kiln. If you have a digital kiln, program a cooling cycle from the peak temp down to 1500F at a rate of 175/hr. That will be a good approximation of the cooling rate in a big kiln. If you have a manual kiln, then you have to be at the kiln when it shuts off, then lift the Sitter weight, turn the kiln back on, and gently lower the weight so it doesn't kick it off. Then put the kiln on medium for about 3 hours, which will cool it more slowly, then turn it off.