Clear glaze looks yellow

[Judy SA]

I teach a ceramic class at The Salvation Army.  I am using Standard low fire slip which I fire to cone 05.  The bisque looks  white.  My problem is when I use a clear glaze and fire it to 05 or 06 it comes out looking yellowish or creamy.  I am aware of the formula change.  I have tried thinning it with distilled water, and putting on a thinner coat of glaze, and used a different brand of glaze.  Nothing I have tried made a difference.  My supplier has told me that they have not had anyone  else with this problem.  Most of the time there is no problem as my class usually use colored glazes.  But, there are times when they want an item to be white and since we are a non  profit I really don't want to use more expesive white glaze.  Is there anyone that can help me?

[Min]

Sounds like you are using commercial glazes? If so there really isn’t anything except trial and error to find one that is satisfactory to you. Yellow tinge is probably from trace amounts of iron in the glaze. Might be less expensive to buy a white glaze rather than trial and error testing of different clear glazes if white is what you are looking for.
BTW, you can turn a clear glaze white by adding an opacifier, zircopax would be the least expensive one. Would need a glaze sieve, accurate scale and a small amount of testing to do this.

[Judy SA]

Thanks.  It sounds like something I am just going to have to live with.  My seniors don' t know the difference and usually put a luster over the clear glaze.  They are happy.  It is mostly me that knows the difference.  I'll just have to remember to use white glaze.  Thank you so much.  Blessings, Judy.

[Babs]

I use a white porcelain slip on red earthenware. Comes out white.

Could it be the white slip turns Cream at glaze temp?

A small unglazed test pot would sort that line of thought.

[Judy SA]

If I change temperature do I go higher or lower.  Do I change the greenware or the glaze firing?

[Rockhopper]

12 hours ago, Babs said:

Could it be the white slip turns Cream at glaze temp?

A small unglazed test pot would sort that line of thought

11 hours ago, Judy SA said:

If I change temperature do I go higher or lower.  Do I change the greenware or the glaze firing?

To expand on Babs' suggestion:  Before changing the firing temperature (or time), make a small test pot and bisque fire it, then fire it in your next glaze load - without applying any glaze - using the same schedule you've been using, and see if the color of the clay changes.  

When you do this, it might be helpful to snap a pre-fire picture of the pot after it's in the kiln for the glaze firing, and another before you remove anything from the shelf around it - just in-case it's affected by 'sputtering' or off-gassing from glazes on surrounding pots.

 

[Judy SA]

Thanks.  I try that.  

Edited March 4 by Judy SA

[Min]

15 hours ago, Babs said:

Could it be the white slip turns Cream at glaze temp?

Bisque is done to cone 05, glaze is done to cone 06 or 05. Could just look at an unglazed part of the pot to see the difference in the casting slip colour.

[Judy SA]

Could the problem come from firing greenware and glaze together?  Since I don't usually have enough for two separate firings I do one firing with both.

[Min]

Anything is possible, it might be from firing bisque with glaze but my hunch it's less probable than being a slightly yellowish glaze colour from trace iron.