With a little exaggeration, I can put that there are as many limit formula as there are authors on the subject. I stick to those mentioned by J. Hesselberth and R. Roy in their book "Mastering Cone Glazes". This range of limits is also available in the INSIGHT and other glaze calculation software.
What is the use of these 'limit formulas'? As J.H. & R.R. put it: they are useful as guidelines, to make 'stable' glazes with the remark that one can make stable, good glazes who are way out of the limit formula.
My definition of a 'stable' glaze is a glaze that:
- Has no glaze flaws like: crazing, shivering, crawling or dunting
- Is food safe when required
Herewith the J.H. & R.R. Limit Formulas. They give the range of moles in the unity formula for the different oxides:
Oxide range (Seger Unity) J.H & R.R.
K+Na2O 0.1 - 0.3
CaO 0.2 - 0.6
MgO 0 - 0.3
BaO 0 - 0.4 (Cooper & Royle)
ZnO 0 - 0.2
SrO 0 - 0.2
Al2O3 0.25 - 0.5
B2O3 0 - 0.3
SiO2 2.5 - 4.0
What you need is just the transformation of your recipe into the Seger unit formula. In glaze calculations software the limits are given next to it.
So you don't need to make graphs, but I do. Graphs are more meaningful to me than just figures. It is a personal preference.
In the example at the end of the text, SiO2 is divided by 10 to keep the graph in good proportions. The red marker lines show the limits.
The glazes MC6G 1 and MC6G 2 are way out of limits for CaO and below limits for the alkalis. These are nevertheless reputed stable glazes. One can here also conclude they are Alkaline Earth matts.
I document all my work with the topics mentioned in these posts, past, present and future.
I repeat: For information on glaze chemistry, visit my website at:
Next time I will discuss briefly thermal expansion and its role in glaze flaws.