|
Felting
Jan 4, 2016 18:02:09 GMT -6
Post by tileman2 on Jan 4, 2016 18:02:09 GMT -6
Evan:
Ever notice the shapes of those felting crystals at the bottom of your bowl? Elongated, whitish grey - ever compare them to Gords avatar that is a picture of sodium crystals? Being the flat guy, I recognize them- sodium impurities in the ZNO, and silica collect at the lowest point and make a nice little nest. Cook em at higher temps and they get that nice blackish color to them.
Gill- proud of ya. You withstood the heckling of all us old coots.
Tom
|
|
|
Felting
Jan 13, 2016 13:22:31 GMT -6
Post by evan cornish-keefe on Jan 13, 2016 13:22:31 GMT -6
Hey Tom, Off the top of my head, I don't recall seeing elongated white crystals in any of my glazes. I've seen loads of rutile crystals which are usually yellow and needle shaped. The glaze on the bowl in that video has no zinc in it, iron and titanium seem to be the crystal forming oxides, that bowl has 15% RIO added, as I reduce the iron (or titanium) I get fewer crystals and less felting. Here's the bottom of a bowl with an RIO addition of 10%. Also, it seems like the clay-body has a very significant effect on felting. I'm still planning to test clay recipes and hope to get a sense of how the number of crystals relates to how vitrified the clay is, or not. Eventually.
|
|
|
Felting
Jan 13, 2016 20:58:59 GMT -6
Post by tileman2 on Jan 13, 2016 20:58:59 GMT -6
Evan:
Been playing around with clay recipes for just over a year. Trying to reformulate to a more crystal friendly recipe. Decided to call in the big guns-booked a meeting with Ron Roy in KC in the middle of March. The Nceca show is the middle of March, and he along with several professors will be there. Might as well pick their brains. A professor from Wyoming will be there, she graduated from U of I back in 73, doing a thesis on crystalline glaze. Only a few hours from me, a mini vacation. Personally I have had many problems with sodium impurities in ZNO, SiO2, and some modifiers. I do not have any felting problems now. Got it out of the ingredients by switching to high purity, now I would like to get it out of my clay. ( if possible.)
Tom
|
|
|
Felting
Jan 26, 2016 7:59:30 GMT -6
Post by evan cornish-keefe on Jan 26, 2016 7:59:30 GMT -6
I hadn't considered sodium impurities as a cause of felting, I usually associate more sodium with fewer crystals? Any advice for a cone 6 clay for crystalline glazes?
I wish i could make it to nceca. A number of my past professors (Matt Katz and Bill Carty) are presenting about glaze safety and durability, they know a whole lot about clay formulation also.
|
|
|
Felting
Jan 26, 2016 21:34:26 GMT -6
Post by tileman2 on Jan 26, 2016 21:34:26 GMT -6
Evan: As I recall, you are already using a French process ZNO-- white zinc. So the purity levels are above 98%, that being true would mean not enough sodium to cause problems. If you would switch to Imsil A-25 SiO2, it would solve the only remaining source of high levels of impurities. I sent some samples of A-25 to a couple of peeps on here: both commented they noticed differences in clarity at cone 10. I have tested around 10 or so varieties of SiO2- Imsil A-25 is by far the best silica out there at nearly 99% pure A-phase quartz silica. In the pic you posted above: those golden and bluish clusters are primarily from sodium. Sodium has a cubic crystal structure, so it tends to form needles and spike crystals. Remember that sodium and potassium can form crystals without any ZNO being present. Buy just a 1/4lb of Imsil and burn that same recipe as shown above... put it to the test. If you like it and see the difference, let me know. I have several hundred lbs. will sell you some for far less. I do not want to get into the selling business- but will help you out to get you started. I also use A-25 in my clay- noticed the difference in that as well. Tom
|
|