Substituting Materials by Weight: Why it does not work!

Wollastonite is 50:50 CaO:SiO2. So why not just substitute 40 wollastonite for 20 calcium carbonate and 20 silica?

D. Desktop Insight


Click here to watch this at youtube.com or click here to go to our Youtube channel

Consider a puzzling question using Insight.
Double-click wollastonite. Has 1:1 CaO:SiO2
Show recipe window with recipe 1 as 20 whiting, 20 silica, 40 feldspar, 20 kaolin; recipe 2 as 40 feldspar, 20 kaolin, 40 wollastonite.
I have it in Insight in place of Calcium Carbonate and Silica. But CaO goes up, while the SiO2 goes down. Why?

We are dealing with a viewpoint issue: we weigh material powders, but formulas compare molecule numbers.

Oxide dialog demonstrates a key point:
We know what each oxide does in a fired glaze.
But we relate the degree of influence to their #s
So 10 grams of Li2O is going to do alot more fluxing than 10 grams of BaO!
SiO2 and CaO are about the same: But these are oxides, not materials.
K2O/Na2O same effect in glazes, but by #s only
BaO/SrO: Similar fluxing power by molecules, not weight.

Another dynamic: LOI (compare Wolly:Whiting carbonates in MDT dialog) Even if I could deem the powders to contribute equal amounts of CaO, this messes up that assumption.

Yet another dynamic: These formulas recalculate to flux unity. Changing the amount of any oxide changes everything. It is like percentage in recipe.

To level the playing field when substituting materials we use no-unity.
Change a material supplying one oxide: only 1 changes

With noUnity lets compare the recipes
(CaO is way up, SiO2 up a little, expected)

So, lets do this right and substitute 20 calcium carbonate for wollastonite.

Notice that to replace 20 whiting I need 23.4 wolly and to reduce the silica by 12.
Need to retotoal.

Have you seen the other tutorial videos on this at http://digitalfire.com/university/index.html?

Out Bound Links

In Bound Links

  • (Glossary) Analysis

    Conceptually we consider fired glazes as being composed of 'oxides'. Materials supply those oxides to the melt. The ten major oxides likely make up 99% of all base glazes (and materials we use). The oxide formula of a glaze "explains" many details about the way the glaze fires. Thus we can view m...

  • (Glossary) Formula Weight

    Quite simply, the weight of a formula. Typically, in glaze chemistry, when we refer to formula weight it is assumed we are talking about the weight of the fired formula of a glaze (without LOI and volatiles). However is is possible to also talk about the formula weight of a material (although materi...

  • (Glossary) Formula

    Conceptually we consider fired ceramic glazes as being composed of 'oxides' (materials contribute these). The ten major oxides likely make up 99% of all base glazes (and materials we use). The oxide formula of a glaze "explains" many details about the way the glaze fires (provided all the materials ...

  • (Glossary) Material Substitution

    Material substitution is a constant and ongoing part of any ceramic enterprise that is using clay and/or glaze recipes. Often lack of availability, quality issues and price are the motivating factors. In addition, when recipes need to be used in another locality where the same raw material brands or...


By Tony Hansen




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