Substituting Nepheline Syenite for Soda Feldspar

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

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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? Let’s see.
Let’ 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.

Out Bound Links

  • (Videos - Related) Lesson 1B - Turning a Feldspar Into a Glaze

    Shows you how to consult a target formula and compare it with the chemistry of a feldspar to determine why a feldspar does not make a good glaze by itself and what materials need to be combined with a...

  • (Videos - Related) Lesson 1A - Comparing a Theoretical and a Real-World Feldspar

    This video is packed with information. Transcript: Video 1C - Comparing a Theoretical and a Real-World Feldspar 1 Entering recipes, materials and the materials database, theoretical materials,...

  • (URLs) Tutorial Videos at Digitalfire

  • (Materials) Nepheline Syenite - Generic

    Neph Sy

  • (Materials) Soda Feldspar - Na2O.Al2O3.6SiO2

    Na-Feldspar, Na Feldspar, Albite

  • (Glossary) Feldspar Glazes

    Quite simply, feldspar glazes are high in feldspar. Feldspar by itself melts well at high temperatures, however to be a balanced glaze (durable, well fitted to the body, non-leachable, etc) it needs additions of other fluxes and silica. It is very educational to work through the process of comparing...

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...

  • (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...

By Tony Hansen

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