Poor results with Amaco Potter's Choice Blue Spark (PC-11). What can I do to improve next time?

[MKG001]

I applied 3 coats of Amaco PC-11 Blue Spark to a piece (light-colored Blick Stoneware Clay bisqued to ^04) and fired it at ^6. Far from giving the desired lapis lazuli-like blue, it turned a deep, shiny, pitted green, as shown in the photo. 

My witness cone on the bottom shelf looked maybe a bit above temperature, but not much. Unfortunately I forgot to put one on the top shelf where this piece was located. So maybe it was just a bit too hot, but I'd think a glaze rated cone 5/6 would have been okay.

I'd be grateful for advice on how to do better with this glaze :-)

Thanks!

Mike

[Roberta12]

I have to ask, what is the firing range on the Blick Clay?   Got a bit of pitting there, along with not being blue.

 

[MKG001]

Thanks, it's ^5-10, so should be robust! https://www.dickblick.com/products/blick-stoneware-clay/

The bottom of the pot is nice and smooth, so the pitting must be in the glaze.

[Dick White]

It appears that the pits go all the way through to the body, so the bubbles are starting there. You say the body is rated for cone 5-10. That means it is immature at cone 5/6, not mature until 10. An alleged  wide firing range is one of the unfortunate fallacies perpetrated by the clay industry. My guess is that there is still stuff outgassing from the body at cone 6. If you are using a kiln with a digital controller, you can try a controlled cooling to allow the glaze to heal over any bubbles that are coming up.

As for the unexpected color, I'm not familiar with that glaze.

[Babs]

Have you used this glaze before successfully?

[Kelly in AK]

Magical cone 6-10 rated clay creates many problems while solving only one: You don’t know which clay to pick, so you choose all-in-one. (You can imagine where I stand on this). Okay, two- the kiln master now has no worries about melted pots in c10 firing. 

Until someone offers proof that a clay body can be vitreous (<2% absorption from c.6 to c.10), and not suffer from excessive warping or bloating in its stated upper range, and not suffer from glaze defects related to outgassing in its lower range,  I’ll be calling BS on the “wide firing range” bodies.

They have applications, but they’re specialized.

 I strongly agree with @Dick White, that clay will be off gassing at various rates the whole time from bisque to six. The bisque firing will not eliminate that. Some glazes will handle it well, others, not so much. A drop and hold/soak schedule might solve it. Still, at the lower range of your clay you may have some pots that leak.

Check the “absorption data” from the clay seller. 

[PeterH]

52 minutes ago, Kelly in AK said:

Check the “absorption data” from the clay seller. 

https://www.dickblick.com/products/blick-stoneware-clay/
Bisque fire to Cone 04 (1971°F, 1077°C). The firing range is Cone 5–10 (2205°F–2381°F). When fired at the recommended temperature of Cone 5 (2205°F), the hard, dense, semi-vitreous ware is creamy tan color with a shrinkage of 12% and absorption of 1.8%. At Cone 10 (2381°F), the very dense, vitreous ware is a soft gray color with a shrinkage of 13.2% and absorption of 0%.

[neilestrick]

So it's actually a cone 8 clay. Not as tight as I'd like at cone 5, too tight at cone 10 (for a stoneware body).

[MKG001]

Thanks all!

@Babs I haven't used this glaze successfully before.

@Kelly in AK @PeterH @neilestrick Thanks for the insights about the clay-- very educational for me.  I bought this clay because it was conveniently available and affordable, not because I envisioned going to cone 10.  Fortunately, my next box of clay is a Laguna cone 5, so it will be interesting to see how that does.

[neilestrick]

In looking at other examples of that glaze, it appears that it does like to go metallic. The lack of color may be due to thickness issues. I'd run a few test tiles ranging from 2-5 coats of glaze and see which one gives the best color. If they all stay metallic then it's probably a temperature issue.

[MKG001]

Would outgassing of the clay also cause this kind of pitting?

This is same clay, same kiln-firing, different glaze. 

[Min]

28 minutes ago, MKG001 said:

Would outgassing of the clay also cause this kind of pitting?

Yes. Other factors involved but body outgassing is definitely one.

[Callie Beller Diesel]

If the pitting is showing up on the same clay but with a different glaze, that’s a strong indication the clay body is part of the problem.

An interesting observation: I went to the website link Peter posted to see if I could find a more detailed MSD sheet. I wanted to see if there was any mention of any material that might be off-gassing. When I zoomed in on the pictures of the glazed bowls, that glaze also shows signs of pitting. So that’s a pretty strong indication.

I didn’t look too hard for the MSD sheet after that, but I’m pretty confident there’s an inexpensive ingredient, or one that makes the clay easier to use that has a lot of sulphites or other organics. If you’ve got any of this clay left and you’re doing your own firing, you might try using the slow bisque cycle if you’re not already. If you are, program a bit of an extra 15-20 mini soak in the 250-600 C/ 480-1110 F range to allow some more burn off time. 

[PeterH]

12 minutes ago, Callie Beller Diesel said:

If you’ve got any of this clay left and you’re doing your own firing, you might try using the slow bisque cycle if you’re not already.

Would bisque-firing twice have a similar effect? It might be an easier test to perform.

[MKG001]

@Callie Beller Diesel That's really interesting.  I also see the pitting in the glaze bowls on the Blick page for this clay!  Now I'm wondering if some ingredient (sulfites per your suggestion) might be altering the chemistry of the glaze.

Oh, I found the MSDS: https://www.dick-blick.com/msds/DBH_SDS_305171050.pdf The only listed ingredients are as follows:

Edited May 31, 2023 by MKG001

[Dick White]

There is another theory about pinholes in commercial brushing glazes that is not widely discussed. That theory holds that when the first coat of glaze is brushed on, there are likely to be some small voids in the coating that are nearly unnoticeable. When the second coat is applied, it doesn't get down in these little voids, creating air bubbles where the second (and later coats) cover them up. Late in the firing, the melt finally gets down to that layer, and the bubbles erupt out through the glaze. Just an idea. If you don't have trouble with this clay body pinholing with dipping glazes, you probably can eliminate the body as the problem and look towards your brush application.

[MKG001]

First, I want to thank everyone who replied for generously sharing your expertise!  

Second, in case you are interested, here is what I heard back from Amaco customer support, after sharing details of the clay body, firing schedule, etc etc:

Quote

Micheal, first when the blue turns green it indicated that the glaze is not thick enough,  How did your test tile turn out?  June bug and Blue spark are difficult glazes to get right.  Normally I use a liner glaze like hf-9 or hf-10 for the insides of my stuff.  and I try to do a 5 cone instead of a 6. 

To save this one you could fire it again at cone five and see what happens.  You could dab a little glaze in the pin holes to fill them.  Since the are so deep I think the clay was still off gassing during the glaze fire. 

I was surprised by the idea of refiring at lower cone (5) to potentially recover, but I'm game to try it.

[MKG001]

Hm customer support added that this glaze seems to do better with a clear liner glaze inside the pot. Mysterious, but I'll try it.

[Babs]

3 hours ago, MKG001 said:

Hm customer support added that this glaze seems to do better with a clear liner glaze inside the pot. Mysterious, but I'll try it.

Well inside a functional pot would be better for hygienic  reasons and to get a stable glaze where lips and food touch, it won't do anything for your problematic glaze.

Could try dipping the pot in water prior to your first brush on, may alleviate the prob Dick mentioned re 1st coat applic. Or have a thinner pot of glaze for 1st coat so the surface gets fully coated.

Interesting that the producer says it's finicky.

Some filk drop temp after reaching goal temp. and soak to allow blemishes to heal over.  Wouldn't help your elusive blue colour. 

[Callie Beller Diesel]

@MKG001 the MSDS doesn’t give anything that screams out “this clay has a bunch of stuff in it that needs to burn off.” It could be the feldspar, could be they used a plastic but dirty kaolin, could have been another ingredient that isn’t listed, but contributes disproportionately to the Loss On Igniton numbers. 

[MKG001]

Well, for those following this thread... I joined an Amaco ^5/6 glaze group on Facebook and searched for mentions of this Blue Spark glaze there. A lot of people struggle to get it to behave as advertised. Lots of bubbles and pinholes, lots of colors not as expected. To the point that Amaco support posted a document called Troubleshooting Blue Spark Glaze, and one forum member was so frustrated he told Amaco support he is switching to Mayco glazes!  

I'll play around with it more based on insights provided here and also on the Amaco group ... 

Thanks again for all the expert feedback! It's very educational for me.

[graybeard]

Could be out gassing Perhaps  the pitting is caused by the fact that the bisque fire wasn't hot enough to allow the clay to completely out gas so when you glaze fired the remainder bubbled through the glaze leaving the pinholes.

Graybeard

[Jeff Longtin]

I've not used blue spark but I have used Palladium quite a bit. 

I apply 5 coats. (to a 06 bisque pot, porcelain) The first two coats are thin. The remaining three coats are progressively thicker.  I only use bottle thick as the final coat. 

I'm not used to brushing glazes so the bottle thick always scares me. (Along the lines of what Dick mentions.) Bottle thick always seems to leave brush strokes/pinholes so I find the two thin layers prevents that from happening.

[rebekahp]

I’m also trying to get familiar with brush on glazes - by ‘bottle thick’ do you mean applied at the provided consistency straight from the bottle? 
 

or is the thickness you’re talking about all in the application of your glazes?