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Custer Feldspar Substitution


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I am attempting a John Britt oil spot glaze recipe I will use in a cone 6 electric kiln.

I have Feldspar G-200 EU, Minispar 200 and Minispar 250, but no Custer. 

Would someone mind redoing this glaze for me using a glaze program, or just tell me if I can substitute one of the feldspars that I have for the Custer.  Thanks!

The recipe:

John Britt's OIl Spot #2

Gerstley Borate      33%

Custer Feldspar      33

EPK Kaolin                   6

Silica                              28

Zircopax                      11

 
 
 
 
 
 

 

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1 hour ago, lincron said:

I have Feldspar G-200 EU, Minispar 200 and Minispar 250, but no Custer. 

I believe G200 Is being replaced by G200 HP which is slightly different but still worth trying IMO these are all so close that some simple glaze calc manipulation should be doable in the event that the one to one does not work out.

comparison below FYI

E36ACF87-5F8C-4849-BF8C-F37C42049928.png

Edited by Bill Kielb
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Both of those are potash feldspars -custar and G-200eu

In the potters world of feldspars there are soda spars and potash feldspars I'm keeping it simple.

Most you can swap around but keep the potash potash or the soda soda if possible . If you do swap them entirely you will need to redo the formula  most likely 

for some of old timers like me  potash feldspars are Kingman-Custar and G-200. I have a supply of them all.

In the mining world they go thru materials and companies get bought out and close down the mines to sell other products that Co. mines. Its a starnge corparate world out there.

Best to buy a large supply to handle all the ups and downs

Edited by Mark C.
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17 hours ago, Bill Kielb said:

I believe G200 Is being replaced by G200 HP which is slightly different but still worth trying IMO these are all so close that some simple glaze calc ...

Some history and clarification about the G200 series feldspars - G200 was (the important word: "was") a potash spar roughly equivalent to Custer at the time it was in production. The G200 mine in Monticello, GA began to run out in the early 2000s, but they were able to keep the brand going by using a feldspar from another mine in Siloam, GA (about 50 miles distant). That feldspar, however, was considerably higher in its potassium content. They resolved the chemistry by trucking a soda feldspar in from Spruce Pine, NC (about 250 miles distant, probably from a mine near the Minspar source and the now-closed Kona F4 source mines) and blending it 70:30 Siloam potash spar:Spruce Pine soda spar. In about 2009, the company decided it was getting too expensive to truck both feldspars for processing and blending in Monticello, so they announce to their customer base that henceforth they would sell only Siloam product with the higher potassium, now labeled as G200HP. They revealed that they had been blending it for years and customers could either blend it themselves with Minspar or recalculate their glazes to the higher potassium content of the G200HP.  Lauguna, for example, began mixing them in their own facility and selling it as "Old Blend." However, as happens in the world of mined products, the Siloam mine ran out in about 2013, and G200HP is now unavailable. To meet the demand for a potash spar, they began to import a potash feldspar from Spain that was comparable to the original G200, and labeled it as G200EU. This product remains currently available, but not as widely carried by distributors and a bit more expensive due to the transport costs from Spain.

At the same time, Laguna found a potash spar in India that is very similar to the original (and blended) G200 and imports it under the name Mahavir feldspar.

Shifting now to the Custer issue, the original G200 and the blended G200 were roughly equivalent at the time, and often subbed 1 for 1 for each other in glaze recipes with no problem. When potters began to realize the bag of G200 they had so blithely just picked up from their distributor was in fact G200HP (the bag and label coloring were similar, only the printed name with the additional letters "HP" was the give-away) and their glazes were overfluxing, some did their own blending but then changed to using Custer. But then people who had long been using Custer began to notice their glazes were underfiring. After some potters sent their Custer out for testing at independent labs, it was found that the Custer product was now actually significantly lower in potassium and alumina and higher in silica than advertised. Pacer Corp, the producer of Custer, still claims in its technical literature that the analysis is basically the same as advertised some 20 years ago and blames the problem on customer (the potters) misuse. In my experience and similarly reported by others, if you have an old recipe containing a significant amount of Custer (40%+), you might need to recalculate your recipe, or change to G200EU or Mahavir.

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@Kaolinwasherhad an analysis of Custer  done in 2017 and posted the results in this thread. Analysis comes out as:  72.47 silica 15.24 alumina, .27 Iron. .25 calcium , .08 MG,  3.45 sodium, 7.42 K20  0.01 titanium .26 phosphate.

Taken just now is the screenshot below from Pacer for 200 and 325 mesh Custer.

486085776_ScreenShot2020-09-02at12_08_41PM.png.85c2e76ec126bb707d827f6a17f10a20.png

 

 

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Thanks Dick,

G200EU is what I used in place of the Custer.  I've not fired it yet, so time will tell if there are any issues.  I only made a small batch to test and will make sure I get a good result before batching up. Thanks for all that info and your time and trouble to post it for me.

Edited by lincron
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this thread made me run out to the studio and read all my huge, old rubbermaid bins of various feldspars.  i have custer which is also labelled  (potash), Kona F4  (soda) and a half shoebox full of G200 (potash).   all of these have been here for more than 20 years.   i cannot find. and i don't think i ever bought Minspar but there is another shoebox full of something unhelpfully labelled soda feldspar.  it looks lumpy, not floury.

should i just put the G200 into the larger Custer bin and mix them together?  there are probably 25 pounds of the custer and i weighed the G200,   it is 6 pounds. 

 i have seen recipes calling for Minspar.   what should i use in it's place?  

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Kona F4 was (again, "was" is the important word) a soda feldspar mined in Spruce Pine, NC. A bit before the turn of the century, there was an unfortunate fire in the mine, and it was uneconomical to repair and reopen. End of Kona F4. There was another company mining a soda feldspar on the other side of the same mountain. That was named NC-4. For whatever marketing reasons, the company selling NC-4 decided to change the name to Minspar. So, if you ever run out of that historic bag of Kona F-4, Minspar is almost the same. In some areas of the world, the feldspars are typically not sold by brand name, but by generic type. Soda feldspar, F4, NC-4, Minspar, all about the same and mostly interchangeable with each other (after testing to be sure exactly how whatever you just got works in that particular recipe).

Potash feldspar is the flip side of soda. Feldspars typically have both sodium and potassium in the percentage analysis, and whichever of them is higher gets the name attribution.  Potash spar is the generic term for spars that have more potassium than sodium. Custer, G200, G200HP, G200EU, and Mahavir are US brand names for spars that lean to the potash side in varying proportions. If your Custer is fairly old, it is similar to the G200 and I would just mix the small amount of G200 with the Custer and not worry about it.

In the interest of completeness, but not really part of this specific question, spodumene is a feldspar that has lithium among its alkaline fluxes. If you need lithium and can't pay the current highway robbery for it, try recalculating your recipe to use spodumene. And finally, there is nepheline syenite. From the geologist's perspective, it is not a true feldspar, but is close enough to be called feldspathic. Names, schmanes, whatever. It is a flux material very high in sodium, higher than proper soda feldspars.

carry on as you were,

dw

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I went with Petalite for adding some lithium to a clear glaze that still crazes - trimming back higher expansion oxides - it was cheaper than spodumene, but has less lithium oxide as well; chose it over spodumene on account of the foaming/washing accounts, and (if I'm remembering correctly) the lithium is less soluble (solubility at issue due to a) lithium is a drug, and b) accounts of lithium causing both crazing and shivering due to how well it moves to the edge o' the clay body...).

Any road, if spod is a feldspar, is Petalite also?

...will post updates to https://community.ceramicartsdaily.org/topic/19922-clay-composition-and-crazing/ , have one round of lithium addition to report one, round two pending...

 

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7 hours ago, Hulk said:

Any road, if spod is a feldspar, is Petalite also?

 

Yes, petalite is classed as a feldspar. Petalite is more often used in clay bodies than glaze, so that's why I didn't include it in my comments about feldspars in glazes. But you can use it if you have it. As for the foaming/washing issue with spodumene, a soap is used during the crushing and processing of the ore, and traces of it remain on the final product. When mixed with water in the glaze slurry, it will generate its own miniature bubble bath, which can cause problems as the glaze dries on the ware leaving bubble voids which become pits in the fired glaze. The soap can be eliminated by washing the spod first in a bucket of water and then drying it. I find that a messy process. An alternative is calcine it to 5-600℉/~300℃ to burn off the soap. I keep tubs of calcined EPK, ZnO, and spod on hand for use as needed.

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Since Aardvark still carries Custer Feldspar, I'll go there on Tuesday and get some.  I'd rather not risk using the G200 EU now that I have seen that melt test.  Although I am firing to cone 6, still better to be safe than sorry.  I hate wasting materials and the time and trouble to mix, test, fire etc.

 

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8 hours ago, Russ said:

g200eu vs. Custer. Very noticeable difference

Careful with  color comparisons In melt and fluidity tests. Check out Custer Vs. G 200 here. https://digitalfire.com/picture/vaxlunogop Custer does not look nearly as dark as your picture above. The earth geologically is cone ten, feldspars basically melt there. Some melt more than others and have somewhat greater fluidity. Most melt less than folks anticipate actually.

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