Test Formulas

For my semester project, I plan to look at the coloration, crazing, and surface effects that various feldspars have on blue celadons.  In particular, I want to determine the effects of varying the amounts of potassium and sodium.  I also hope to increase potassium levels until my celadon has the opalescence found in blue chuns.

I loved two of the glazes on my initial and secondary test tiles (see my previous post): #12 and #18.  I know, I wrote about #11 and #17, but after closely examining my second batch of tests, I like #12 and #18 better.  #12 is a nice buttery satin, and #18 is a gloss.  Both pooled nicely into the tile pattern, so I look forward to seeing what they will do on larger pieces.

I was somewhat dismayed at the amount of colorant that the glaze pulled out of the white stoneware, so I'm going to have to use a smaller amount of iron for that clay (Dan's Four Equal Parts) if I want it to match the results I get on grolleg porcelain.  It looked like the clear glaze pulled about a quarter of a percent's worth of iron out of the clay, so initially I'll use .25% less (based, of course, on a standard 100% formula) than what I use on the porcelain.

The problem with swapping out feldspars -- or any other natural product -- is that they aren't pure.  That means that a gram-for-gram swap will vary fluxes and colorants so much that the new glaze will not be viable.  I spent six hours yesterday trying to find feldspar/grolleg/silica/whiting ratios that, when combined, have the same levels of fluxes as the original formula. 

I've been using a piece of software called Insight, a particular piece of genius designed to save ceramicists long hours in the lab.  It creates an on-the-fly Seger formula of any combination of ingredients, as well as determining the thermal expansion rate.  This last is particularly valuable, especially since it will also calculate the thermal expansion of any clay body.  Matching these rates will allow me to avoid many of the fit problems (primarily crazing) that plague celadons.

First I looked at feldspars that have large discrepancies between amounts of potassium and sodium.  Here are the Seger values for the ones with the largest differences:

Potash Feldspars

• Custer -- K .676,  Na  .291
• K-200 -- K  .652,  Na  .329
• G-200 -- K  .638,  Na  .274
• Oxford -- K  .589,  Na  .364
• Eureka -- K  .638,  Na  .361
• Bell --  K  .653,  Na  .304

Soda Feldspars

• NC-4 -- K  .243,  Na  .611
• Nepheline Syanite -- K  .214,  Na  .692
• Kona F-4 -- K  .263,  Na  .574
• F-20 -- K  .244,  Na  .616
• DF Cornwall Stone -- K  .378,  Na  .589
• SP-G -- K  .244,  Na  .616

Next, I plugged each of the base formulas into Insight and started swapping out the feldspars in turn.  I wanted the new formulas to be as similar to the original formula as possible, so I manipulated the amounts of individual components until the Segers matched.  I did what I could to keep the thermal expansion under 8.0, but I was more concerned with matching the Segers than matching the clay at this point.  I'll worry about that another day.

I wanted to isolate a single variable in this series of tests: the ratio of potassium to sodium.  There is a setting in Insight that allows the user to consider the ratio as one combined total, KNaO.  I chose that setting so that the Seger value of potassium and sodium together was equal from one formula to another.  Each feldspar has a different amount each, of course, but that is the purpose of this test.

Here are the recipes that I will be testing, assuming that I can get my hands on all of these feldspars.  I'll post photos of the results.

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MATTE GLAZE 12

Custer Feldspar........ 45.50 Whiting................ 19.50 Grolleg Kaolin......... 24.17 Silica................. 10.83 Add: FeO............... 0.50 ========= 100.50	K-200 Feldspar......... 43.74 Whiting................ 19.58 Grolleg Kaolin......... 24.41 Silica................. 12.26 Add: FeO............... 0.50 ========= 100.50
G-200 Feldspar......... 44.02 Whiting................ 19.62 Grolleg Kaolin......... 24.18 Silica................. 12.18 Add: FeO............... 0.50 ========= 100.50	Oxford Feldspar........ 51.97 Whiting................ 19.56 Grolleg Kaolin......... 21.44 Silica................. 7.02 Add: FeO............... 0.50 ========= 100.50
Eureka Feldspar........ 45.59 Whiting................ 19.74 Grolleg Kaolin......... 24.42 Silica................. 10.25 Add: FeO............... 0.50 ========= 100.50	Bell Feldspar.......... 45.27 Whiting................ 19.40 Grolleg Kaolin......... 24.05 Silica................. 11.27 Add: FeO............... 0.50 ========= 100.50
NC-4 Feldspar.......... 46.91 Whiting................ 18.86 Grolleg Kaolin......... 22.60 Silica................. 11.63 Add: FeO............... 0.50 ========= 100.50 Note: NC-4 is now called Minspar.	Nepheline Syenite...... 34.44 Whiting................ 19.52 Grolleg Kaolin......... 24.18 Silica................. 21.86 Add: FeO............... 0.50 ========= 100.50
Kona F-4 Feldspar...... 44.59 Whiting................ 18.66 Grolleg Kaolin......... 23.13 Silica................. 13.62 Add: FeO............... 0.50 ========= 100.50	F-20 Feldspar.......... 47.00 Whiting................ 18.90 Grolleg Kaolin......... 22.55 Silica................. 11.56 Add: FeO............... 0.50 ========= 100.50
DF Cornwall Stone...... 36.62 Whiting................ 19.73 Grolleg Kaolin......... 22.78 Silica................. 3.47 Nepheline Syenite...... 17.39 Add: FeO............... 0.50 ========= 100.50 Note: Cornwall has so much silica, I had to add NephSy to offset the imbalance.	SP-G Feldspar.......... 47.39 Whiting................ 18.86 Grolleg Kaolin......... 22.12 Silica................. 11.63 Add: FeO............... 0.50 ========= 100.50

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GLOSS GLAZE 18

Custer Feldspar........ 42.88 Whiting................ 18.38 Grolleg Kaolin......... 16.25 Silica................. 22.50 Add: FeO............... 0.50 ========= 100.50	K-200 Feldspar......... 40.88 Whiting................ 18.50 Grolleg Kaolin......... 16.71 Silica................. 23.90 Add: FeO............... 0.50 ========= 100.50
G-200 Feldspar......... 40.70 Whiting................ 18.04 Grolleg Kaolin......... 16.22 Silica................. 25.05 Add: FeO............... 0.50 ========= 100.50	Oxford Feldspar........ 48.43 Whiting................ 18.43 Grolleg Kaolin......... 13.97 Silica................. 19.17 Add: FeO............... 0.50 ========= 100.50
Eureka Feldspar........ 43.11 Whiting................ 18.56 Grolleg Kaolin......... 16.41 Silica................. 21.92 Add: FeO............... 0.50 ========= 100.50	Bell Feldspar.......... 42.75 Whiting................ 18.32 Grolleg Kaolin......... 15.90 Silica................. 23.03 Add: FeO............... 0.50 ========= 100.50
NC-4 Feldspar.......... 43.85 Whiting................ 17.92 Grolleg Kaolin......... 14.21 Silica................. 24.02 Add: FeO............... 0.50 ========= 100.50	Nepheline Syenite...... 32.01 Whiting................ 18.45 Grolleg Kaolin......... 16.31 Silica................. 33.23 Add: FeO............... 0.50 ========= 100.50
F-4 Feldspar........... 41.27 Whiting................ 17.69 Grolleg Kaolin......... 15.16 Silica................. 25.89 Add: FeO............... 0.50 ========= 100.50	F-20 Feldspar.......... 44.05 Whiting................ 17.85 Grolleg Kaolin......... 14.41 Silica................. 23.69 Add: FeO............... 0.50 ========= 100.50
DF Cornwall Stone...... 64.77 Whiting................ 18.56 Grolleg Kaolin......... 15.42 Silica................. 1.24 Add: FeO............... 0.50 ========= 100.50	SP-G Feldspar.......... 44.37 Whiting................ 17.87 Grolleg Kaolin......... 13.80 Silica................. 23.96 Add: FeO............... 0.50 ========= 100.50