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Cobalt Spreads To Other Glazes In Kiln?

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My third glaze firing has produced better, but slightly unexpected results...

 

Just opened my small electric kiln to find that the plain white glazes are speckled blue. The effect is not unpleasant, but it isn't what I planned for. 

 

Question: I had some cobalt blue glazes in the kiln and I think their strong blue colouring must have spread around the kiln?

 

Blues mostly were on bottom shelf and the most affected whites were on that shelf and the one immediately above it. The top shelf showed least contamination.

 

Question: Some of the ware may have been damp from glazing when it went into the kiln – might this have exacerbated the spread of blue through excessive water vapour?

 

Question: Some pin holes have also appeared in glazes in some places – due to moisture again maybe?

 

Question: one of the pots, the glazes have clumped together and some have run off the bottom of the pot in places. The pot was thin walled and quite damp from glazing the inside when I glazed the outside, I don't think the glaze 'adhered' very well to the damp clay – would this have exacerbated the slumping? This pot is shown in the second photo below.

 

The attached photos, the left area on the first pot and the lower areas on the second and third pots is the 'white' glaze that shows the blue speckling/contamination.

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post-61816-0-79317500-1394877336_thumb.jpg

post-61816-0-21776300-1394877819_thumb.jpg

post-61816-0-14631000-1394886611_thumb.jpg

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You've got some crawling going on in the middle photo. Applying the second dip while the first was still wet is likely the culprit.

 

I wouldn't expect the cobalt to vaporize to the extent that you're mentioning. Also, it usually doesn't speckle, but rather blushes blue. It will typically only happen if the white piece is right next to the the blue piece. It looks to me like your white glaze is a titanium white, applied very thickly. What you're getting is titanium blue, which can happen with certain glazes that are high in titanium. It's probably also the cause of the pinholes. Also, if the white is applied on top of the blue (can't tell if this is the case), the blue will come through and discolor the white.

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I agree with Neil. This looks like a glaze application issue.The middle photo the glaze looks to be twice as thick as it should be. Cobalt is a very stable colourant and does not normally fly around the kiln like copper would. It's a strong colourant. You could be contaminating your wares with your hands/utensils.

Pin-holing can come from dust or too rapid a rise in glaze temperature.

Ain't ceramics grand? Give it 20 or 30 years and you will be O.K.

Tom.

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Thanks for the useful information. I didn't know about 'titanium blue' – I seem to learn a lot from these 'errors'.

 

Next glazing I will try less titanium in the white glaze (I'll probably half the amount of titanium dioxide) and I'll allow the pots to dry between coats and before firing.

 

I'm also going to halve the amount of cobalt oxide, aiming that the intended blue areas will be paler ( I never wanted such a strong blue).

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The titanium will give nice crystal growth, but if you just want white, opacify your glaze with zircopax or superpax instead. It will be whiter and avoid discoloring.

 

Thanks Neil.

 

What I do like is the reaction between the titanium and cobalt glazes where they overlap - gives a kind of blotchy blue/green effect which is the best thing I have so far managed to make : )  I guess that is crystal growth???

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I love the "overlap" effect.   I aim for that on my mugs, white gloss inside and around the outside of the rim, and the bubbling, dribbling effect where the two meet.  Yummy. 

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