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Do Glazes "spoil"?


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#1 Pam S

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Posted 16 April 2017 - 08:50 PM

My favorite white glaze has gone weird on me. It was made about 2 years ago and last used in September. I tried to use is night before last and noticed a change in the texture when I was stirring. It felt slimy. When I dipped a clean damp pot in it the glaze immediately started to flake off. I took it to the sink to wash the glaze off the pot. When water hit it the glaze it congealed (felt like sheet rock mud) and was hard to wash off the pot. Any thoughts?

 

White Silk c5-6

 

Dolomite 60 g

EPK 226 g

Flint 143g

GB 135g

Kona F4 300g

Neph Sy 592g

Talc 97.5g

Whiting 128g

Zinc Oxide 116g


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#2 Min

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Posted 16 April 2017 - 09:41 PM

Sounds like your glaze has deflocculated. The F4 plus the Nepsy supply sodium (amongst other stuff) to the glaze, both those ingredients are slightly soluble. Over time the sodium (alkali) cause a glaze to defloc.  I would scoop out a couple cups of glaze and add some epsom salts solution, just a tiny bit at a time, give it a good stir, let it sit for a couple minutes and see if the glaze slurry stays suspended or starts settling out. If it does the latter then add a tiny bit more and continue doing so until it stays suspended. Don't overdo the epsom salts or you just run into other problems. Dip a test tile into the now flocculated slurry and see how it behaves on the tile. Hopefully all is good, in which case fix up the bucket of slurry in the same way. So, to answer your question, no it's not spoiled just deflocced. 



#3 glazenerd

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Posted 16 April 2017 - 10:01 PM

Was bentonite added by chance to help suspend this glaze? Every ingredient posted could sit in a bucket for 100 years without doing anything but bleed off some salts: except the EPK. Any clay suspended in water will have a slimey feel due to chemical reactions with alkali ( salts). If bentonite has been added; even more slimey. The flaking off is caused by two things: high PH levels due to all the alkali fluxes; coupled with magnesium from the dolomite and talc. Personally I would not add more magnesium (Epson salts), but rather take out a half cup of glaze and replace it with vinegar. The acidity of the vinegar will lower the high alkali PH caused by the fluxes: which is causing the glaze to act hydrophobic. ( causes rapid drying=flaking). Any potter who has mixed high magnesium glazes for lava for instance, can attest to how flaky it is. Do glazes spoil? No.  Do they change characteristics over time? yes. Lowering the PH will also alleviate the slime feel.

**Vinegar is organic and will burn off completely when you fire.

Nerd



#4 Pres

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Posted 16 April 2017 - 10:37 PM

Most instances as nerd says non organics do not do much and I would say no. However, if you glaze freezes, I would sieve it twice through the same sieve you used the first time. Sometimes they come out more grainy after freezing, sieving will take care of that. . . in my experience.

 

 

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Pres


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#5 Mark C.

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Posted 17 April 2017 - 12:00 AM

One other note is did any water evaporate off in the last few years? How are you measuring the glaze as to thickness? Hydrometer or weighing or hand dip test? Since the glaze flaked off was it to thick?

I would follow Mins post and see what happens. I have never in 45 years had a glaze spoil on me.

I do not let them freeze ever.


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www.liscomhillpottery.com

#6 Min

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Posted 17 April 2017 - 06:20 PM

@ Pam S, there is a super article on Adjusting Glazes for Application, which covers flocs and deflocs here. Can use different materials to floc a glaze, epsom salts, vinegar, calcium chloride, muratic acid (this one is a bit dangerous though), each has it's merit.

 

@ Nerd, did you mean a reticulated (crawl or lichen) glaze here instead of lava? "Any potter who has mixed high magnesium glazes for lava for instance, can attest to how flaky it is."  



#7 glazenerd

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Posted 17 April 2017 - 09:26 PM

Yes Min, crawl glaze- TY.   In this case, I think this glaze has a serious case of hydrolysis. When soluble salts separate their cations and anions releasing hydrogen: the PH  goes way up. Same thing happens in a water softener: you get that "slime" feeling in the water. 



#8 Pam S

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Posted 18 April 2017 - 06:44 PM

Thanks for all the good advise. The article was particularly helpful in wrapping my head around things. Community lessons and other classes. The chemistry of it all is a bit daunting. I have about 3 gallons of this glaze so I can try several of the solutions and see what works best. Again, many thanks!

 

BTW, the glaze never froze. It is stored in my home.


"Saving just one dog won't change the world, but it surely will change the world for that one dog."





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