Substituting Nepheline Syenite for Soda Feldspar

Learn to substitute Nepheline Syenite for Soda Feldspar (and vice versa) using the KNaO concept in Insight. You will see the benefit of in-recipe substitution calculation rather than making general substitution rules.

D. Desktop Insight

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Issues surrounding substitution with complex materials that supply many oxides, KNaO vs. K₂O/Na₂O evaluation, substitution rules vs. in-recipe substitution calculation, non-chemistry reasons for glaze differences and flow testing

Substituting Nepheline Syenite for Soda Feldspar
Welcome. In this lesson we are going to learn to substitute a feldspar for nepheline syenite.
We will discuss the two materials and the issues surrounding substitution with complex materials, KNaO vs. K₂O/Na₂O evaluation, substitution rules vs. in-recipe substitution calculation, non-chemistry reasons for glaze differences and flow testing

Comparing feldspar and nepheline
mFeldspars and Nepheline Syenite are similar from a chemistry point of view and it is common to substitute one for another on a one-to-one-weight basis. Is that a good idea? Lets see.
Let's start with 100 parts F4 feldspar in recipe 1 line 1 and 100 of Nepheline in recipe 2 line 2 and RO Unity.
Notice how much different these two materials are with respect to the amount to SiO₂ and even the KNaO.

Settings: KNaO, No-unity, .000 accuracy
I have the KNaO checkbox checked to combine K₂O and Na₂O amounts, using feldspar I need to be content to match the total of the two.
I am going to set both recipes to No Unity calculation, that way I can build a duplicate that will not need to be retotaled.
I also have the lessons materials database selected as with other lessons and am going to set three decimals accuracy since I am dealing with smaller than normal formula numbers.

Match KNaO
I have started by matching the KNaO.
I could not use the Supply button because it is a combination of two oxides. So to match I clicked the KNaO line to be able to keep an eye on it and then selected the Nepheline Syenite line and made sure recipe 2 was selected.
Then I clicked the decrement button until the KNaO numbers matched. They matched at 72.5 parts of Nepheline.
Notice as I got closer I changed the increment by to 0.5. I did that quickly by holding down the control key while clicking the down arrow to cut the By amount in half.

Supply the CaO from wollastonite
The CaO is a lot easier. I am going to source it from Wollastonite because I need both SiO₂ and CaO and it supplies both. I need 0.03 of CaO.
I clicked the next blank line, entered part of the word wollastonite and pressed the key. INSIGHT looked it up and filled in the whole name for me. Now I will click the Supply button.

Establishing the wollastonite amount
The Supply dialog appeared and I have clicked the CaO button. It knows I need the .03 by looking at the formula of the other recipe. I will click Done.

Supplying Al₂O₃, SiO₂
Now here is the recipe and CaO and KNaO match.
Now I just need to match up the Al₂O₃ and SiO₂. I will do this using the same method to supply Al₂O₃ from Kaolin and SiO₂ from silica.

The result shows how different NephSy is
Here is the result, a matched formula and a recipe whose total is so close to 100 I do not need to bother retotaling it.
When you look at the combination of materials needed to match the chemistry of Nepheline Syenite it becomes evident how different this feldspar and nepheline syenite are and why people who simply do a one-to-one weight substitute often run into problems.

Substitution in other direction?
While I can now specify a simple F-4-to-Nepheline substitution rule it is impractical to do so for the other direction.
Notice I have matched the KNaO for 100 parts nepheline, that requires 138 F-4. But this oversupplies alumina and really oversupplies silica. However, like the last lesson of substituting calcium carbonate for wollastonite, you could formulate a complex rule that involves removing wollastonite, kaolin and silica from the target recipe. But it is possible that one of these will not be present.

Substitute in-each-glaze is better
In situations like this it is actually much easier to put the recipes side by side with one having the old feldspar and the other the substitute. Then juggle the new recipe to match up the KNaO and then the other oxides. When you do it this way there are more options and you can think about improving other aspects of the recipe at the same time.

Feldspar flow tests show differences
How different are feldspars. For interest sake, these melt flow tests compare four different feldspars. It is pretty evident that they have different melting properties, especially these two. These differences can be accounted for partly by their chemistry, but not only that.

Non-chemistry reasons for differences
Ceramic chemistry is a relative science and it is one piece in the glaze puzzle. You cannot ignore mineralogy and physical factors like particle size. These two glazes have the same calculated chemistry but they do not melt the same, they do not have the same gloss and one has many more entrained bubbles. However they employ different feldspars to source sodium and potassium.

Links

URLs	https://digitalfire.com/videos Tutorial Videos at Digitalfire
Materials	Soda Feldspar
Materials	Nepheline Syenite
Media	Desktop Insight 1A - Compare Theoretical and Real-World Feldspars While comparing a real-world and theoretical feldspar learn to enter, edit, save, normalize recipes and the materials dialog. Glaze chemistry concepts.
Media	Desktop Insight 1B - Turn a Feldspar Into a Glaze Learn to compare a target formula with the chemistry of a feldspar. See why it does not make a good glaze by itself and what materials need to be added to make it into a balanced glaze.
Glossary	Feldspar Glazes Feldspar is a natural mineral that, by itself, is the most similar to a high temperature stoneware glaze. Thus it is common to see alot of it in glaze recipes. Actually, too much.
Glossary	Material Substitution Material substitutions in ceramic glaze and body recipes must consider their chemistry, mineralogy and physical properties
Glossary	Digitalfire Insight A downloadable program for Windows, Mac, Linux for doing classic ceramic glaze chemistry. It has been used around the world since the early 1980s.

Cannot get Nepheline Syenite? Here is what to do.

Nepheline Syenite is similar to a feldspar. I have them open side-by-side here in my Insight-live.com account (the blue panels). In the "Analysis" column notice that Minspar has 9% more SiO₂ and 5% less Al₂O₃. Minspar has 12% fluxes and Nepheline has more than 15%. If a recipe does not contain a significant percentage of Nepheline these differences might be tolerated but what if there is 30% or more? There is no combination of materials that has the chemistry of Nepheline (there is no way to take 10% of the SiO₂ out of this feldspar, for example). But it is possible to take SiO₂ out of a glaze containing Nepheline Syenite. Notice the two green recipe panels below: The changes made to the one on the right harmonize the oxide chemistry with the original on the left. Those changes were significant: 15% more feldspar, 12% less silica and 2% less kaolin. Notice how easy this was using a little chemistry!

By Tony Hansen
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