Best substitute for G-200, Minspar or Nepheline Synenite + Silica?

[MFP]

I have just used the last of my G-200. I have seen several suggestions for a substitute but would like everyone's opinion.  Thanks!

[Bill Kielb]

Custer is fairly similar. I would look up YOUR G200 in a materials  database such as on Glazy.org  (https://glazy.org/materials/56699) and pick the next similar feldspar available to you locally. Neph sy is not similar in my view (soda feldspar vs potassium). If you use glaze calc, or the Glazy calculator then the glaze recipe can be tweaked to match its original composition as close as practical with the available substitute feldspar.

[MFP]

Thanks, Bill. Digital fire says minspar but then another source thought Nep Syn + silica would be better. Since I use it in an overglaze, I need it to be melty. Digital fire comments on Neo Syn being the most melty of the spare.

[Bill Kielb]

51 minutes ago, MFP said:

Digital fire comments on Neo Syn being the most melty of the spare.

If the recipe melts with G200 then use Custer or minspar. Neph sy is a soda spar, G200 is more a potassium spar. Compare materials to keep the recipe the same, not whether something melts earlier. I think I would pick (my opinion) Custer, then minspar and very last neph sy just rough comparing which are most similar in chemical composition.

 

[Min]

Post the recipe and we can try to tinker it to match the formula using an alternate spar or nepheline syenite. 

[MFP]

50. G-200

50 Gerstley Borate

40 Silica

10 Zircopax

5 EPK

2 Bentonite

[MFP]

It's an overglaze. I got the recipe from glazy and it has worked very well for me. Better than Vickry overglaze or John's Mino.

[Leaman Pottery]

Love, love this! Thanks for the recipe and photo!!!

[MFP]

Most welcome! Great glaze.

[Min]

I replaced the G200 with custer and adjusted other materials to balance original recipe. Bentonite probably isn't necessary with all that gerstley in the recipe.  Super high boron amount for a durable cone 6 glaze, it's what's giving you the runny / drippy effect.  If you try my adjusted recipe for a 100 gram batch just round the numbers off to the nearest tenth and please try a small test amount before using on actual pots. What looks good in theory isn't guaranteed to work in practice.

 

 

[Bill Kielb]

25 minutes ago, Min said:

Super high boron amount for a durable cone 6 glaze, it's what's giving you the runny / drippy effect. 

If he decides to test this it probably melts nicely at cone 04 but may not have near the movement.

[MFP]

Thank you, Min. I appreciate it. Is M-200 the same as minspar? I tried that as a substitute and it made it a tad stiff.  Actually, it was just an oops. I grabbed the wrong container. Apparently all I saw was the 200.

Since you have to glop it heavily along the rim of the pot--- that's what you need the bentonite for. It has to be super thick. It also keeps it from falling off the pot onto the shelf!

[MFP]

Bill-- it does not melt at 04. Full cone 6 is required to get that result.

[Bill Kielb]

2 hours ago, MFP said:

Bill-- it does not melt at 04. Full cone 6 is required to get that result.

Hmm, it’s got enough boron to melt at 04 for sure. You have tried firing it at different cones? What is the glaze name on Glazy for this?

[Min]

12 hours ago, MFP said:

Is M-200 the same as minspar?

I would assume that's what it is but double check with the studio. Another thing I would check on is if the G200 feldspar you originally used was G200 or G200HP, makes a difference as the latter has  higher potassium levels than former G200. If you want to try and make it even runnier try it without the epk, it might cause crazing though.

 

[MFP]

The G-200 I used was 45 years old.  ( I still have Kingman). 

[MFP]

Yes.  It melts but stays in a solid lump. It doesn't flow.  And that's at 5. It does have all that silica. I grabbed the wrong container once and made it with M-200 and it was even stiffer. I had to add more Gerstley borate to get it to flow.

[MFP]

It also depends what it's sitting on. The glaze in this picture is a celadon that matures from 4-6. If it's on a stiffer glaze, it melts even less.

[Bill Kielb]

28 minutes ago, MFP said:

It also depends what it's sitting on

Ahh, I get it you are rim dipping these over an existing glaze. I think it’s still very likely this melts on its own at cone 04 or so. Just something you may find useful at some point. What recipe is this on Glazy?

[MFP]

No, I put huge globs of the glaze all along the rim. At least 1/2 inch thick.  It was just titled Gerstly Borate Overglaze. It was right before or after the Vickry oil spot and overglaze recipes. It's better than Vickry.  It's also better than John's mino which has a tendency to form crisp bubbles. This overglaze has worked well on every type of glaze I have used it over. It's very versatile.

[Min]

Is this the original recipe from Glazy? If it is then your version of it that totals 157 above isn't quite the same formula wise when both are totalled to 100.

 

 

[MFP]

Yes-- this is the original recipe from glazy. It's just titled Gerstley Borate Overglaze. I have used this recipe as written for 2 years. I noticed it didn't add up to 100. I also thought the smidgen of EPK was a bit strange. But it's been a great recipe.

I know you will cringe but when I realised I had screwed up and pulled the wrong 200 off the shelf, I just added 1/4 cup of Gerstley to it and it fixed it.

I don't know how this new glaze formulation works. I understand it when it's just the chemical compounds but I don't get the leap to the different glaze materials from those compounds.