Cone 6 Glaze For Sourdough Crock

[sdm1299]

Because of the nature of sourdough, I want to be sure the clear, cone 6 glaze that I am using is safe.  Following is the recipe ---
Spodumene  35%
SGP-1 (ball clay) - 12%
Whiting - 17%
Flint - 19%
Ferro Frit 3134 - 17%
Bentonite - 6%
 
I hope someone can give me some insight on the safety of this glaze and a suggestion if you do not think it is safe.  Thank you for your help.

[Min]

Hi sdm1299 and welcome.

 

That’s a fair amount of spodumene, lithium level is going to be getting up there. Don’t know if it will leach or not, only way to know for sure is to get it lab tested. That being said the silica and alumina levels look okay, not spectacular but just okay, in particular the silica level could be higher. You want both the alumina and silica levels to be on the high end of those limits for a really durable glaze. I don’t know what sourdough starter would do to a glaze so can't comment on that.

 

Any particular reason you are using this recipe for a liner glaze? Looks like it might be a pain to work with having that much spodumene. Do you get soapy bubbles from the spodumene? I can see why you have to add 6 bentonite to get it to stay in suspension.

 

Spodumene, or something else to supply lithia, is usually used to bring down the expansion of a glaze. The expansion on this one looks to be in the “average†type of range so I’m just wondering why you would be using a glaze with such a high level of it for just a clear with middle of the road expansion glaze? There are more user friendly clear glazes out there. 

 

The column on the left is your recipe, the one on the right is "target limits" for a ^6 glaze. (so you can see what I'm talking about re silica and alumina)

[sdm1299]

Thanks for the help.......Not being a chemist, I use glaze recipes that have been developed by others.  I just started using this one and stayed with it in the absence of a replacement. It is hard to keep in suspension so I will look for one that is more user friendly.  Again, I appreciate your help!!

[Min]

Maybe try posting another thread asking for a clear liner glaze? There are some clears in the book Mastering Cone 6 Glazes. You wouldn't have to worry about leaching with one of those.

(I would give you mine but it's a low expansion glaze for porcelain) 

[glazenerd]

 

Spodumene  35%                                   13.0  

SGP-1 (ball clay) - 12%                          12.0                

Whiting - 17%                                           10.0                                   

Flint - 19%                                                 27.0  

Ferro Frit 3134 - 17%                              15.0

Bentonite - 6%                                            0.0

                                                                     23.0   Nep Sy.

Alumina 4.58  (5.0 max limits)     Silica 3.96  (4.0 max limits)     Lithia (um) 0.126  ( no formula limits) but found 0.100 up to 0.150 works very well.

 

Remember, this is a clear; so a careful eye has to be kept on iron, magnesium, and titanium have to be kept ultra low: any of the three can color, cloud, or effect clarity.   MGO 0.005   FE  0.004   TiO2  0.006   (ultra clear )   COE 6.81

 

One concern: all of the oxides,silica, alumina, and modifiers are within limits: but at the high end of limits. The combination of lithium, high KnaO, and frit may still cause glaze run. Do a spare piece first.

 

Nerd

[sdm1299]

Thank you for this redo! I'm anxious to try it.