Jump to content

What Is Causing The Black Zits In My Glaze?


Darcy Kane

Recommended Posts

so I have been working along making the switch from ^10 to ^6 and overall things are going ok.  However, the Waterfall Brown glaze from MC6G came out with little black bumps.  It is on Standard 112 bisque fired to ^06 and fired to ^5.  Would it have been more successful if I had bisqued to ^04?  I like the glaze enough to mess around with it a bit.

post-9888-0-79860200-1411772295_thumb.jpg

post-9888-0-95406200-1411772303_thumb.jpg

post-9888-0-85947800-1411772378_thumb.jpg

post-9888-0-79860200-1411772295_thumb.jpg

post-9888-0-95406200-1411772303_thumb.jpg

post-9888-0-85947800-1411772378_thumb.jpg

Link to comment
Share on other sites

Standard 112 has bits of manganese in it -- which gives the clay body its speckled brown look. Those flecks show through most glazes I've seen on the clay body. You likely have something in the glaze that is reacting with the manganese. Changing bisque temperatures will not help. You might need to use a different clay body, something without chunkies.

Link to comment
Share on other sites

I kinda like the look of them. Are they a tactile problem?

For some I think they would be.  Not for me because I like to "worry" at things with my fingers, but other people would just look at it as a flaw (not my OCD, the bumps :) ).  They stick out like a piece of sand under the glaze.  Next firing I will try it on another clay body and see if it does better. I like the black speckling too but not the way it raises out from the surface.

Link to comment
Share on other sites

After reading the comments I remembered that I had used another glaze over the 112 so I went and compared results.  The two mugs attached show speckling but no raised zits.  I also had a couple of mugs where the waterfall brown glaze wasn't applied heavily enough to get the better result and those two don't have raised bumps, so I think it is safe to deduce that the fault is in the combination of glaze and clay body.  I will have to try a few more combos as I do like the glaze.  Everything can't be blue and green!

 

Babs, the foot and rim on that mug show what the glaze looks like where it is thin.  It is not an attractive skinny glaze!

 

post-9888-0-34290000-1411824402_thumb.jpg

post-9888-0-89683600-1411824410_thumb.jpg

post-9888-0-34290000-1411824402_thumb.jpg

post-9888-0-89683600-1411824410_thumb.jpg

Link to comment
Share on other sites

The second set of mugs look like Standards 112 with the usual speckling. On the first set, where were these in the kiln? Are the unglazed bottoms of a different color/value when comparing the two? I just wonder if you had a hotspot in the kiln, as the first set looks like they are slightly over fired.

Link to comment
Share on other sites

The first mug (the brown one) was on the top shelf, but I only went to ^5 when the clay and glaze is supposedly ok to ^6.  And the bottoms are the same color on both the blue and brown mugs.  I also put mugs with different glazes on all three shelves for better comparison and this was the only one with zits.  

Can you help me see why it looks over fired?  In my limited experience, over fired always meant on the kiln shelf  :D

Link to comment
Share on other sites

i scrounged around and found another piece that may help.  This one was on the same shelf as the first but had a thinner coat of glaze.  In fact, only where it concentrated in the middle was actually thick enough to get a reasonable result.  You can see the speckling but no black zits, however it also doesn't have the same runny look as the first mug.  Thickness of glaze seems to be the key, both to the zits and they effect I am looking for.

post-9888-0-37184900-1411828863_thumb.jpg

post-9888-0-37184900-1411828863_thumb.jpg

Link to comment
Share on other sites

Were the black zit pots near a thermocouple? Could this possibly be spalling from a thermocouple?

 

btw, you really got a nice result from that glaze overall. I tried making Waterfall Brown once and didn't have nearly as nice of a result.

Link to comment
Share on other sites

If fired to ^5, the over firing is not the problem. I have fired on occasion to ^6.5 and 7 with the 112, and would get surfaces more in line with what I saw with your glaze in first set.  the third photo shows normal speckling from the 112, so I would say you need to experiment with test tiles of varying thickness of the waterfall to see where you are happiest with the glaze.  112 has been my go to clay for many years as it throws well, handbuilds well, and has a texture that I like in feel and fired look.  I have used it for many years along with a white or cream clay for a different palette with the same glazes.

Link to comment
Share on other sites

 112 has been my go to clay for many years as it throws well, handbuilds well, and has a texture that I like in feel and fired look.  I have used it for many years along with a white or cream clay for a different palette with the same glazes.

 

It was partly on your recommendation that I tried the 112 and overall I like the results I got.  I will have to mess around with it a bit more as it seems that the issue is the thickness of the glaze.  Of course I prefer the appearance of the thicker glaze, just not the zits.  Didn't you mention that 112 is also available without the speckles/mag? Perhaps that would be a good way to avoid the zits but still get a darker foundation clay. I too like to use a whitish clay and a darker clay so it varies my color palette and my glazes do double duty.  

 

Mea, it may have been near a thermocouple.  I will have to take that into consideration next time.  this glaze is finicky for sure.  Not enough and it is UGLY, too much and it runs like a mad bastard, but it is pretty enough for me to keep working at.

 

And I did screen the glaze before using it.  

Link to comment
Share on other sites

225 is basically the very same clay without the inclusion of the granular manganese. I used this clay for many years in the HS I taught at. I started using the 112 when I started buying clay for myself. This way there would be no question as to where I was getting my clay. . . . called covering your back. :)

Link to comment
Share on other sites

I used 112 for a while, finally gave up on it. I just couldn't get constant results with it. I would have some beautiful pots, then I would have pots where the specs created little holes in the inside of the specs. I tried everything over 10 firings to solve the problem never could get it down. Your pots look pretty good though, congrats on getting it to work for you.

Link to comment
Share on other sites

I use Waterfall regularly, on various clays, including Highwater's Speckled Brownstone, and don't have any troubles with it having those zits.  I fire it hotter than you do, to a tip down ^6 and sometimes hotter, no bumps, and a different color,  brighter amber brown with lots of variation, blues, reds and the black edging if it fires just right. I have another glaze that want to have a cottage cheese look sometimes on ^6 porcelain, but not bumps.

Link to comment
Share on other sites

I use Waterfall regularly, on various clays, including Highwater's Speckled Brownstone, and don't have any troubles with it having those zits.  I fire it hotter than you do, to a tip down ^6 and sometimes hotter, no bumps, and a different color,  brighter amber brown with lots of variation, blues, reds and the black edging if it fires just right. I have another glaze that want to have a cottage cheese look sometimes on ^6 porcelain, but not bumps.

That is very hopeful.  There is no reason why I can't fire to a hot ^6.  Would you mind posting a few pictures of your waterfall brown results?

Link to comment
Share on other sites

Archived

This topic is now archived and is closed to further replies.

×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.