Asking for a friend: underglaze question

[Callie Beller Diesel]

A mutual on instagram, who happens to live next to the ocean, has recently had problems with her underglazes getting more fluid in the bottle over time. Particularly the Velvet ones.  As a firmly landlocked person who lives in a semi-arid climate, I have never experienced this in my life. Usually we get the opposite. I wondered if it was possible that  the brushing agents/gums in the  UG’s were possibly absorbing water from the environment, and a quick google search does show that CMC gum is indeed hygroscopic. But that’s a very cursory search on my part, and I don’t know for sure what else is in there, or if CMC is even what they’re using. 

So this question is for the folks who live in humid environments: have you found your underglazes going kinda thin? Particularly the Amaco Velvet line? Does anyone know if there are other possible ingredients that would absorb water from the atmosphere?

[Min]

Does the underglaze smell at all? Thinking the gum(s) are breaking down.

[Roberta12]

My orange and red velvets get thin after a longggggg time.  In the pints.  The cost has gotten so prohibitive, I am no longer ordering pints, except for white.  And the Flame orange does have that sulphur smell.  after researching last year, I came to the conclusion that @Min stated, it's the CMC breaking down.  But I do not live in a humid area, nor is my studio humid.  Quite the opposite.  I live in the Sahara of Colorado.  Maybe humidity hastens the process.  

[PeterH]

Some previous thoughts below.

Especially @neilestrick
I agree that the organic binders/extenders are diminishing. Typically they put a biocide in there to preserve them, but if they are especially old it can wear out. Add some gum solution with a touch of copper carb in it to preserve it. I usually make gum solution 2 tbsp bum to a gallon of water, plus 1/8 teaspoon copper carb.

As your friend's underglaze seems to be having  a shortish life before thinning is there any special source of contamination/warmth that might be causing this? Are they using large bottles, and might decanting a working-stock into a smaller bottle help? (And maybe keeping the large bottles in a cooler location.)

 

Edited yesterday at 10:30 AM by PeterH

[Min]

If the solution is to add gum then I would make as concentrated a solution as possible to add to reduce the amount the underglaze is diluted as much as possible.

[Callie Beller Diesel]

Thanks all! She said the UG’s do indeed smell like sulphur. She will try adding gum.