Formulating Substitution Rules for Calcium Carbonate and Wollastonite

How to use desktop Insight to substitute wollastonite for calcium carbonate (and vice versa) while maintaining the same SiO2 level. Create substitution rules.

D. Desktop Insight

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How to use Digitalfire Insight software to determine how much wollastonite to substitute for whiting to maintain the same SiO2 in the glaze. Covers rationalization of the relative merits of materials, using the Supply Oxide dialog both ways on side-by-side no-unity recipes, and learning to spot obvious supply problems and theoretical vs. actual materials.

Welcome. In this lesson I am going to talk about formulating material substitution rules, I will use calcium carbonate and wollastonite as an example.
We will be discussing rationalization of the relative merits of materials, using the Supply Oxide dialog both ways on side-by-side no-unity recipes, and learning to spot obvious supply problems and theoretical vs. actual materials.

Comparing these materials
As you might know, wollastonite and whiting (or calcium carbonate) are two common sources of CaO used in glazes. You can find out more about them the Digitalfire Reference Database.
Theoretically, whiting is a mix of CaO and CO2 gas, it has a high LOI and is inexpensive and potentially inconsistent, wollastonite is a mix of CaO and SiO2, has no LOI and is more expensive and more consistent.

How to substitute for wollastonite?
Suppose you are asked what proportion of whiting and silica should be put into a glaze to substitute for a wollastonite amount of 30 (we will consider a whiting-to-wollastonite switch in a moment). This problem is different than described in the previous lesson because you have not been given the recipe. There are some mistakes that are very easy to make here so let us go through it and see.

Add silica & fine tune its amount
I have set both recipes to RO Unity.
And entered 30 wollastonite for recipe 1 line 1.
And switched to recipe 2 and entered 15 whiting in line 2 and 15 silica in line 3 (I am guessing on the amounts).
INSIGHT calculates this formula.
Now I am going to click the decrement button until the SiO2 figures in the formula match. That happens at 9 parts silica in the recipe.

Problem: Set to no unity
Take a closer look.
This cannot be right because whiting has a high LOI (it loses weight during firing as some of it turns to gas) so the combination of silica and whiting raw powder must total more than the amount of wollastonite. The problem is that I should be calculating with a No Unity setting for recipe 2 so that the whiting/silica formula represents the actual weight of material needed, not the relative numbers of molecules. I will do that now.

Match the no unity formula
I have just increased the whiting and silica weights until both the CaO and SiO2 numbers in the formula match.
But this cannot be right either, how could 160 parts of whiting:silica be needed to substitute for just 30 wollastonite?
What is the problem? Recipe 1 is still set at RO Unity. Remember, unifying a recipe means that changing the total of a recipe has no effect on the calculated formula, it is always unified. But I need the formula to be sensitive to the weight I specify.
I will set recipe 1 at No Unity calculation.

Use the supply button
Although I could move the amounts of whiting and silica up and down using the increment and decrement arrows, there is an easier way.
I will select recipe 2 and click the whiting line. I will click the Supply button to tell INSIGHT how much CaO I want from whiting. The supply dialog defaults the number of CaO oxide molecules being sought to the number in the other recipe.
I will click Done.
Select the silica line for recipe 2 and click the Supply button and do the same.

That looks better, now we have a match. It takes 25.8 grams of whiting and 15.51 of silica to substitute for 30 of wollastonite.

Formulating a substitution rule
Suppose I wanted to come up with a general guide line such as "for each gram of wollastonite substitute x grams of whiting and y grams of silica". I have just done it.
I selected recipe 1 and changed the amount of wollastonite to 100
Then selected recipe 2 and used the Supply button to resupply the whiting and silica.
Now I can say: For each 100 grams of wollastonite substitute 86.1 grams of whiting and 51.7 grams of silica, or for each 1 gram substitute 0.86 grams of whiting and 0.517 grams of silica.

Inversing the rule
Now, let’s invert that rule. I have changed the amount of whiting to 100 in recipe 2, selected recipe 1 line 1 and used the Supply button to source CaO from Wollastonite.
Then I selected recipe 2 line 3 and supplied SiO2 from silica.
The formulas now look like this.
So the rule now is: For each 100 grams of whiting substitute 116.2 of wollastonite and remove 60.1 of silica. For each one gram whiting I would add 1.162 of wollastonite and remove 0.601 of silica.

What was the magic?
The magic behind this two-way calculation technique is INSIGHT's dual recipes, its ability to calculate with no unity, its Supply Oxide dialog, and the fact that each recipe is a fully functioning entity.

The End
In conclusion I want to remind you that we have been talking about theoretical calcium carbonate and wollastonite, for two real-world materials with contaminating oxides it would be more complicated. But this general rule should still be sufficient for most situations.

Out Bound Links

  • (URLs) Tutorial Videos at Digitalfire

  • (Videos - Related) Lesson 1C - Substituting Wollastonite for Whiting in Glazes

    This video discusses substituting for and comparing calcium carbonate (whiting) with other sources of CaO (dolomite, wollastonite, frit): It teaches you how to go about using glaze chemistry to unders...

  • (Materials) Wollastonite - CaSiO3 - Calcium Silicate


  • (Materials) Calcium Carbonate - CaCO3

    Carbonate of Lime, Whiting, Aragonite, Calcite, CaCO3

  • (Glossary) LOI

    Simplistically, LOI is the percentage of weight a material loses on firing. Assuming firing to a typical stoneware temperature of 1200C, the amount of weight loss can be surprising. Kaolins, for example, lose around 12% (mainly crystal-bound water). Ball clays lose about half of that (a combination ...

In Bound Links

  • (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...

  • (Videos - Related) Substituting Materials by Weight: Why it does not work!

    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...

By Tony Hansen

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