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? The answer will help you see reason why we make such a big deal of glaze chemistry.

D. Desktop Insight

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

Let's consider a puzzling question using desktop Insight. Transcription of this video follows.

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?

Links

Media	Desktop Insight 1C - Substitute Wollastonite for Whiting in Glazes Compare calcium carbonate (whiting) with other sources of CaO (dolomite, wollastonite, frit), learn to understand the chemistry differences between materials and then substitute wollastonite for whiting in a specific recipe.
Media	Creating Rules for Calcium Carbonate - Wollastonite Substitution How to use Digitalfire Insight software to determine how much wollastonite to add and silica to remove to substitute for calcium carbonate in the glaze. Create substitution rules.
Glossary	Material Substitution Material substitutions in ceramic glaze and body recipes must consider their chemistry, mineralogy and physical properties
Glossary	Oxide Formula In ceramics, the chemistry of fired glazes is expressed as an oxide formula. There are direct links between the oxide chemistry and the fired physical properties.
Glossary	Formula Weight
Glossary	Chemical Analysis In ceramics, raw material chemistry is expressed a chemical analyses. This is in contrast to fired glaze chemistries which are expressed as oxide formulas.
Glossary	Glaze Chemistry Glaze chemistry is the study of how the oxide chemistry of glazes relate to the way they fire. It accounts for color, surface, hardness, texture, melting temperature, thermal expansion, etc.

By Tony Hansen
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