Lesson 1C - Substituting 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.

D. Desktop Insight

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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 understand the differences between materials and then remove whiting and replace it with the preferred material, wollastonite, in a recipe. The reason for doing this, in this case, is that wollastonite is a better quality and more consistent material and it is not a carbonate, so it does not produce gases as it decomposes during melting (these gases can cause issues with glaze bubbles, pinholes, blisters, etc.).

An important point to remember about materials substitution, wollastonite for whiting in this case, is that you cannot just replace one with the other gram-for-gram. Whiting supplies only CaO oxide to the glaze melt whereas wollasonite supplies both CaO and SiO2. In addition, 1 gram of wollastonite does not supply the same amount of CaO as 1 gram of whiting. Thus, to substitute, you need to calculate how much wollastonite is needed to supply the same amount of calcia, but also how much the silica in the recipe needs to be reduced so the overall SiO2 content in the glaze stays the same (becase the incoming wollastonite is bringing its own SiO2). This process is quite fascinating and opens up the doors to being about to substitute many other materials; ultimately you will be able to work a frit into a recipe (frits are premelted and generate no gases during melting, they are the ultimate material to substitute many oxides that are delivered by troublesome raw materials (eg. MgO in talc or dolomite, Li2O in lithium carbonate), even substitute more than one at the same time (e.g. removing both Gerstley Borate and whiting from a recipe and supplying the CaO and B2O3 from a frit).

Transcript of video 1C - Comparing Calcia Sourcing Materials

LOI and study calcia contributing materials specifically, what makes frits such great sources of oxides, the concept of alternate sourcing an oxide, non-participant recipe materials and the digitalfire material information strategy

Two Sources of Calcia
Welcome. In this lesson we will talk about LOI and study calcia contributing materials specifically. We will discuss what makes frits such great sources of oxides, the concept of alternate sourcing an oxide, non-participant recipe materials and the digitalfire material information strategy.

Examining calcium carbonate
We are going to take a closer look at the makeup of calcium carbonate (also called whiting). To more fully understand this material it needs it to be alone in a recipe.
I will select the custer feldspar line in recipe 1 (from lesson 1B) and click the Delete button to remove the amount for the feldspar.
This leaves only the whiting in recipe 1.
Before continuing I should point out that the calculated molecular weight of calcium carbonate is 100, by a rather amazing coincidence. Hopefully that will reduce a little confusion in this lesson.

Why is the formula weight different?
Notice the formula weight INSIGHT calculates for whiting in the Calculated Items list: '56.1'.
If I double-click the whiting recipe line the materials dialog shows a weight of '100'. Why? Whiting powder is not pure calcium oxide, it contains carbon dioxide. Of 100 grams of calcium carbonate powder that you put in the kiln, only 56.1 will be present in the fired glaze (the rest is lost as CO2 vapors).
Here you can see that INSIGHT calls the 43.9% loss LOI (literally loss-on-ignition or weight lost on firing). Here is the other 56.1 that is left.
Of course if I double-click CaO to see it in the oxides dialog: Its weight is 56.1.

How INSIGHT handles LOI
When INSIGHT calculates it is portraying only what is left after firing and gives us the weight of that. There is no CO2 in the formula list, it is gone.
So the materials dialog is representing raw materials with LOIs, the recipe window is showing us only the calculated products.
However notice that INSIGHT actually displays two different LOI figures in the recipe window, the Calculated LOI and the Imposed LOI. I will demonstrate.

Before going on note that I have the lessons materials database selected as with other lessons.

Imposing an LOI
I have closed the Materials and Oxides dialogs and will choose Override Calculated LOI in the Calc menu.
Notice this window already knows that my raw recipe has a total LOI of 43.9, so I will set it to that.
INSIGHT now calculates the correct weight for the RAW material and the override I just set is displayed as the Imposed LOI. Being able to impose an LOI is important to adding new materials to the materials database.
As you will learn, when materials lose this much weight during firing they generate a lot of gases, this can have an impact on the glaze, especially if the gases are coming out as bubbles after melting starts.

Finding CaO sources: Wollastonite
There sources of CaO without such a high LOI? I will show you in the materials dialog. Notice on the lower left there is a pop-up list where you can select an oxide. I have selected CaO. As you can see it is obviously sourced by many materials. I'll scroll down the list and show you Wollastonite.
Notice it contains equal CaO and SiO2 (of course this is a theoretical material, real world wollastonites have traces of other things and the SiO2 and CaO are not exactly equal). Like a feldspar, getting SiO2 for a glaze this way is better than from raw quartz powder. Notice also this it has no LOI. Want to learn more? Level 2 INSIGHT users can click this button.

Analyzing dolomite
Here is dolomite. This is actually worse than calcium carbonate. It loses 47.6% on firing, leaving 53.4.
This formula weight, 96.4, is thus 53.4 percent of the formula weight of the raw material.
When we talk about LOI we also need to think about when the gases are liberated during kiln heat-up. If it happens after the glaze starts melting, that is obviously not good. The Digitalfire Reference Library has a special area called Temperatures, it is attempting to map the decomposition events from many materials during kiln firing.

Analyzing a frit
Here is a boron frit, notice that it has 20% CaO. If your glaze also needs sodium, boron and silica, and the vast majority of glazes do, a frit like this is a good source of CaO. INSIGHT is a great way to figure out how to juggle things to incorporate frits into glaze recipes while maintaining the chemistry.
The really great thing about this material that no feldspar on earth can offer is this: No alumina. That makes more room in recipes for clays, they are sources of alumina and suspend the slurry and harden the dried layer. In addition, frits generate no gases on firing.

Non-participant materials
A recipe can contain materials not known by INSIGHT, we call these phantom or non-participant materials.
To demonstrate I have entered a line with 50 parts of “SPIDERS LEGS”. The asterisk before it means INSIGHT cannot find chemistry for it. Organic gums are like this, they are added to glazes to impart working properties but contribute no oxides to the fired product, they just burn away.
When doing glaze chemistry we most often like to see a clean formula not cluttered up with colorants or opacifiers. Thus, you will actually want to leave materials like zircon, tin oxide and raw coloring oxides or stains out of your MDT and enter them into glazes the way I have done with spiders legs here.

Which materials included in MDT?
I want to talk for a moment about how we decide what materials to include in the standard list.
Of course, the lessons materials database, the one we are using here is small. But the standard one distributed with INSIGHT normally has about 250 generic and name brand (mostly North American) materials. The MDT section of the Digitalfire Reference Library website explains the logic behind how we determine which materials to include in the starter tables that are available for various parts of the world and specific ceramic industries. There is a page where logged in INSIGHT level 2 users can add to one of the starter databases and customize materials names and even translate them.

Digitalfire materials strategy
This page at the Digitalfire Reference Library is where we focus our material information efforts. These links and this search are the home page for thousands of materials. This link goes to the area we were just looking at and all of these others are interrelated in a material centric reference database. We will be using this resource much more in subsequent lessons.
Video 7A - Formulating Substitution Rules with Whiting and Wollastonite
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

Out Bound Links

In Bound Links

  • (Glossary) Digitalfire Insight

    A desktop application for Windows, Linux, Macintosh that you download and install. Insight is a classic glaze chemistry calculator. -It interactively converts recipes to formulas and back. The main Insight window shows side-by-side recipes and their formulas, you can make chemistry changes to one...

  • (Videos - Related) Formulating Substitution Rules for Calcium Carbonate and Wollastonite

    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,...

  • (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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