What are these little black dots?

[boardbutproductive]

Hi all! Question- About a year ago, I made a mug using velvet underglaze and zinc-free clear. In the past few weeks, I have noticed that the bottom interior of the mug has developed pinholes with little black dots, and the clear glaze has started to look a bit milky. (See attached pic)

The clay body is laguna cone-5 Bmix, fired to cone 5, so it should be vitrified. Any ideas what is going on here? Is it still food safe?

[Callie Beller Diesel]

“Should be vitrified” isn’t “I tested it under my own firing conditions to make sure”.  

Stuff is getting into the pinholes, and possibly under the glaze. There is an ick factor here for sure. If this is just a personal piece, you might try re-firing it to see if the pinholes needed a little more heat work to clear. It’ll also burn out those black spots.

[Roberta12]

17 minutes ago, Callie Beller Diesel said:

“Should be vitrified” isn’t “I tested it under my own firing conditions to make sure”.  

Stuff is getting into the pinholes, and possibly under the glaze. There is an ick factor here for sure. If this is just a personal piece, you might try re-firing it to see if the pinholes needed a little more heat work to clear. It’ll also burn out those black spots.

Callie, if this person puts bleach in the cup and the black spots go away, would that mean the cup is growing something??? 

[JohnnyK]

2 hours ago, Roberta12 said:

Callie, if this person puts bleach in the cup and the black spots go away, would that mean the cup is growing something??? 

If bleaching the spots makes them go away, it is probably mold...

[oldlady]

or coffee.

[Mark C.]

I'm with oldlady coffee in the pinholes is my vote

[Chilly]

Mould or coffee = unsanitary. 

Bin or re-fire.

[shawnhar]

If you look closely you can see hundreds of little blisters. I think they were always there but you have finally "rubbed through" them with  your spoon and stuff can now get in there. If the "stuff" is just coffee then I wouldn't worry too much, but if it ain't coffee stains it could make you sick, so no, not food safe.

Having said that, a rock from the woods is "food safe" if you heat it up enough, just sayin, 450f heat kills everything

[Bill Kielb]

4 minutes ago, shawnhar said:

a rock from the woods is "food safe" if you heat it up enough,

Roughage!

[Pyewackette]

My little purple cup with the blobs at the bottom from runny glaze - the liner glaze crazed but I didn't notice until enough tea had gotten in there to stain the cracks.  However my tea is just tea - no milk, no sugar - so seems unlikely to grow bad stuff.  I wouldn't sell it (well no one would BUY it, but you know what I mean) but I continue to use it - for now.

Eventually I'll have so many not-that-great cups and mugs that it'll get tossed and replaced with something slightly better that hopefully won't craze, like whatever glaze they settle on so we finally have a cone 10 liner glaze that won't craze LOL!

That looks sort of like pinholing, which for some bizarre reason seems worse to me than crazing.  I would toss that anyway.  Not logical - crazing gives you the same sort of exposure to bugs and fungi and mold as pinholes - but I hate pinholes more.

[boardbutproductive]

Thanks all-- I'm not sure if it's coffee or mold, but either way, I'm planning to refire it to see if that helps. I figure either it fixes it, or this mug goes in the bin like it would have anyway.  

Amaco suggested decreasing the HF-9 glaze I used inside from 3 coats to 2, as they said it looks like the glaze pooled here, causing lots of little bubbles that eventually wore through.  I plan to try that going forward, and also to run some tests with the orange underglaze.  Everywhere I used that orange, it turned into a sort of orange-peel texture (no pun intended) after glaze firing. 

[Roberta12]

17 minutes ago, boardbutproductive said:

Thanks all-- I'm not sure if it's coffee or mold, but either way, I'm planning to refire it to see if that helps. I figure either it fixes it, or this mug goes in the bin like it would have anyway.  

Amaco suggested decreasing the HF-9 glaze I used inside from 3 coats to 2, as they said it looks like the glaze pooled here, causing lots of little bubbles that eventually wore through.  I plan to try that going forward, and also to run some tests with the orange underglaze.  Everywhere I used that orange, it turned into a sort of orange-peel texture (no pun intended) after glaze firing. 

I have had that orange peel texture happen with Amaco's Flame orange underglaze and with the intense Yellow as well if I apply too thickly!

[Mark C.]

1 hour ago, boardbutproductive said:

Thanks all-- I'm not sure if it's coffee or mold, but either way, I'm planning to refire it to see if that helps. I figure either it fixes it, or this mug goes in the bin like it would have anyway.  

Amaco suggested decreasing the HF-9 glaze I used inside from 3 coats to 2, as they said it looks like the glaze pooled here, causing lots of little bubbles that eventually wore through.  I plan to try that going forward, and also to run some tests with the orange underglaze.  Everywhere I used that orange, it turned into a sort of orange-peel texture (no pun intended) after glaze firing. 

When those bubbles wear thru that means glass (glaze) came out into fluids most likely witha spoon stirring for example.Not a good situtaion for anyone. If the glaze is to thick toss it as its unsafe for use.  In ceramics we all make mistakes and try to learn from them.It takes a whole lifetime of ceramic mistakes and then in the next life you get to make more-its the learning from them that makes us better potters

Edited April 20, 2022 by Mark C.

[Bill Kielb]

17 hours ago, Roberta12 said:

Everywhere I used that orange, it turned into a sort of orange-peel texture (no pun intended) after glaze firing. 

It’s not uncommon that some underglazes in some colors have a more refractory effect on the overglaze, especially thick applications of underglaze. Your overglaze does not appear to be super melted to me, might just be the picture though or the stiffness of the glaze. Wash underglaze effects often have less effect on the overglaze.

[GestaltArtworks]

On 4/21/2022 at 5:17 AM, Bill Kielb said:

It’s not uncommon that some underglazes in some colors have a more refractory effect on the overglaze, especially thick applications of underglaze. Your overglaze does not appear to be super melted to me, might just be the picture though or the stiffness of the glaze. Wash underglaze effects often have less effect on the overglaze.

Bill, is the orange peel texture what you are calling "refractory"? I've had this happen with Speedball royal blue on a piece with multiple other colors inside/out and only the blues were affected. Seems like you're saying that painting on washed out has better results, yes?

[Babs]

Not sure   but blue for me,  I have to mix a little frit into the  blue underglaze so it stays smooth under the glaze. A little of the glaze if it is a non run clear would also sort this. As it is not all over your blue dec.  Could it be the rough patches are applied thicker when brush fully loaded? Or underglaze not stirred between brush strokes? 

[neilestrick]

Speedball royal blue is a known problem, especially if thick. Their red does it, too. I use their other blues and reds instead.

[Bill Kielb]

@GestaltArtworks Yes, I am saying that and have found other colors in other brands to be difficult to melt over as well. It  looks like black in that picture as well. Our common finding, especially prevalent over heavily applied colors, the color and brands varied. Since we have brush artists who want to layer colors and paint wysiwyg, meaning our final solution was to create a clear glaze with a bit more boron to melt over heavy applications of underglaze.

And yes, we noted painting washes with the same color did not appear to affect the glaze to the same magnitude of heavily painted areas. The picture below is one we posted on Glazy for Marcia’s matte which is a matte glaze with a bit more boron to avoid these issues. Oops! need to add: The sugar bowl below thrown by me, decorated by Marcia and spray glazed by me was created under Madison pottery. It is a good example of some solid color combinations that were difficult to cover with clear glaze but essential to the artist to paint them as such.

 

Edited December 3, 2023 by Bill Kielb

[glazenerd]

Looking closely, I can see pinholes that are clear. I can see others that are black. I will vote coffee.

Tom