Lesson 1B - Turning 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.

D. Desktop Insight

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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 it to make it into a balanced glaze. You will get insight into why so many of the feldspar glazes in the public domain do not work well for some simple reasons.


Lesson 1B: Turning a feldspar into a glaze
Target formulas, feldspar chemistry, thermal expansion, supplying oxides to a formula from a material, getting detailed material and oxide information at digitalfire and oxide and material level thinking that technicians employ to make formulation decisions

Making a glaze from a feldspar
In this lesson I am going to make a glaze from a feldspar.
We’ll discuss using target formulas, feldspar chemistry, thermal expansion, supplying oxides to a formula from a material, getting detailed material and oxide information at digitalfire.com and oxide and material level thinking that technicians employ to make formulation decisions.

Formula & calculated items
Let's focus of the Formula List and Calculated Items list in the Recipe window.
Engineers have long studied formulas and their relationships to melting temperature, hardness, thermal expansion, gloss, resistance to leaching, color, etc.
This knowledge is well documented today, so we can use INSIGHT as kind of a crystal ball into the way glazes will fire in the kiln.
But INSIGHT does not think for us, it does the conversion from recipe to formula. We still need to interpret these numbers.
What should the oxide amounts be? Which oxides should or should not be here? How do they interact? We will answer these questions.

Open a target formula
mLets get started on answering those questions by continuing from Lesson 1A.
If specific oxides and oxide groups contribute specific properties to a fired glaze, it follows that the amounts of each need to fall within certain ranges to get a workable glaze.
Target (or limit) formulas basically show us these ranges.
I can open one to compare side-by-side with these formulas by clicking here.

Finding the targets
The recipe database window appears.
INSIGHT has searched the database for recipes having the word TARGET in the code number. I can do this anytime within this dialog by clicking this.
I have selected the Green and Cooper Cone 6 item and will click Open.

Targets of KNaO
Before continuing remember you need to have the lessons materials database selected here or your numbers will not match mine.
The target formula has appeared in column 3 in the Formula list. I have increased the width and height of the entire window and adjusted column widths of the formula list to display the target and two formulas properly.
You can learn more about limit and target formulas at digitalfire.com.
I am also going to click the KNaO checkbox. This will combine the amounts for K2O and Na2O since these oxides have similar contributions and almost all raw materials that source one also source the other.
Let's zoom in on the formula list.

Analyzing feldspar chemistry
Notice the K2O and Na2O totals are now combined.
By comparing the formulas of the two feldspars with this target it is evident that feldspar-as-a-glaze is lacking CaO (among other things) and has way too much KNaO. Almost all stoneware glazes have much more CaO than K2O/Na2O (to achieve hardness and resistance to leaching, but especially for resistance to crazing).
How could I know that? I will double-click the CaO line.

Learning about an oxide
The Oxide dialog appears.
Now I will click the Info button.
Now my web browser opens at the page at Digitalfire Reference Database website that tells me all about CaO and what it does in glazes and how it predominates as a flux in stoneware glazes and why.

Thermal expansion numbers
Notice also the expansion for theoretical potash feldspar in column one of the Calculated Items list.
This is really high. I use the INSIGHT standard set of thermal expansion numbers, over the years I have found that common stoneware glazes need to be below about 7.5 to avoid crazing. Of course your circumstances might be different, especially the clay bodies you use.
As an experiment try making a glaze from pure feldspar and fire it on a sample at cone 8 or 10, be ready for the worst crazing you ever saw.
Let me show you something else in the oxides dialog. I will double-click the CaO line again.

Comparing oxide expansions
mThe Oxides dialog appears again. Notice the thermal expanson of CaO, it is 0.148.
What does this number mean? Don't worry about it yet, just consider its relative magnitude. Higher numbers mean crazing, very low ones means that a glaze could shiver. Crazing is where the glaze is stretched on the ware and forms a network of cracks, shivering is where it is compressed onto the ware so much that it flakes off edges to relieve the stress.
Now, I will click the SiO2. Wow, that is more than 4 times lower. And Alumina. It is more than twice as low as CaO.
But now look at Na2O. It is more than twice as much as CaO, the highest of any oxide. So in a relative sense, CaO is a low expansion flux compared to sodium (potassium is also really high). That is why high-feldspar glazes often craze, they contain a lot of sodium and potassium.

Sourcing CaO
The major raw sources of CaO in stoneware glazes are calcium carbonate and wollastonite (at low temperatures Gerstley Borate and frits are used).
How do I know that? It was near the bottom of that web page INSIGHT showed me a minute ago. We can see other materials that source alot of CaO here, one of note is Dolomite.
I will now add calcium carbonate (or whiting) to the Custer feldspar to bring it closer to being a glaze.
Calcium carbonate and wollastonite have different advantages and disadvantages as sources of CaO. What are they?

Finding whiting information
I have chosen Edit Materials in INSIGHTs Utility menu and the materials dialog has displayed.
I have clicked the list of materials and pressed the W key. This took me to whiting.
Now I will click the Info button.

Material info at Digitalfire Reference library
INSIGHT has opened to a page about Calcium Carbonate (which is whiting of course) at the Digitalfire Reference Library.
There is a link to Wollastonite also. And also to thousands of others materials right here.

Whiting into recipe
Now, back to the recipe window.
I have selected Recipe 1 and clicked the ‘potash feldspar’ line and keyed “whiting” into Materials Lookup (in place of potash feldspar) and ‘10’ into the Amount field. Click Update.

Line label override
However the recipe still says Potash Feldspar? This is because the Line Label was not changed. INSIGHT only uses the Materials Lookup field to find the chemistry, it displays the Line Label in recipes. When you create a new line INSIGHT automatically fills in the empty Line Label if not supplied. So to fix it I will remove the “Potash Feldspar” line label and click the Update button. INSIGHT will then copy “Whiting” from the Materials Lookup into it.
To learn more about the purpose and interplay between the label and lookup blanks see lesson 1A.

Whiting helps feldspar
Now I have clicked on the ‘custer feldspar’ line, made sure recipe 1 was selected and entered “49” for the Amount and Updated, then I pressed the up arrow key to move the line cursor back up to line 2.
The Formula List shows feldspar by itself on the right and the whiting-feldspar combo on the left. With the whiting addition, feldspar is now very similar to the target formula. The silica, alumina and sodium/potassium are less than half of what they were and the CaO now dominates the fluxes (marked with asterisks).

Analyzing this mix
As you will see, in glaze chemistry we see materials as “sources of oxides”.
Could this two-material glaze work on stoneware at cone 10? Yes, at least it would melt very well. How about cone 6? No, a glaze with this high alumina needs a melter more powerful than CaO, for example boron from a frit. In addition, the thermal expansion is still much too high and crazing is virtually certain, the KNaO is way too high.
Also this two-material glaze has no kaolin or ball clay, that means it will immediately settle in the bucket and powder on drying. Clay is a perfect substitute for feldspar here because it also contributes lots of alumina and silica but not the expansion-increasing KNaO. I will leave it to you to finish this.
As you can imagine, doing all of this without INSIGHT would be difficult indeed.

Oxide and materials levels
The process of turning pure feldspar into a workable glaze in INSIGHT is a fascinating and educational one.
As you can see you need to think on both the oxide and materials levels. What do I mean by that? The formula determines the way the glaze fires, the recipe determines the way it performs in production of the ware. A third level is the mineralogy of the materials, understanding why they exhibit the physical properties they do.
Thus, chemistry is an important, but not the only piece in the puzzle of understanding glazes.

Out Bound Links

  • (URLs) Digitalfire Reference Database


  • (URLs) Tutorial Videos at Digitalfire


  • (Glossary) Target Formula, Limit Formula

    The term 'limit formula' historically has typically referred to efforts to establish absolute ranges for mixtures of oxides that melt well at an intended temperature and are not in sufficient excess to cause defects. These formulas typically show ranges for each oxide commonly used in a specific gla...

  • (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

  • (Videos - Related) Substituting Nepheline Syenite for Soda Feldspar

    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...

  • (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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