I've just taken a pot melt out the kiln and I'm a bit puzzled. It was made up of BE opal white (700gms) and transparent light violet (400gms). However, the violet appears to have turned clear, with only the slightest trace left in the middle. I've used these two colours before without any problem and I'm intreged to find out why its happened. Has anyone seen anything like this before? Any ideas would be much appreciated.
Assuming that the BE violet is colored with manganese oxide, which seems like a reasonable assumption (Lani?), your color problem is probably due to reduction. While manganese oxide is the most common colorant for violet glass, it is very sensitive to oxidation/reduction color changes. Manganese primarily exists in glass in two different forms (valences), Mn++ and MN+++. The trivalent Mn+++ is the one that produces the violet color. The bivalent manganese, Mn++, is essentially colorless. Unfortunately, the violet Mn+++ is less stable in glass than Mn++.
The hotter, longer, or more often glass is melted the more likely it is to lose oxygen and thus convert Mn+++ to Mn++. It could be that for some reason the atmosphere in your kiln was slightly reducing and caused the manganese in the glass to change valences and thereby lose its color. Or you just got it hot enough to change the oxidation/reduction equilibrium and shift most of the manganese to the essentially colorless Mn++ form.
I've just taken a pot melt out the kiln and I'm a bit puzzled. It was made up of BE opal white (700gms) and transparent light violet (400gms). However, the violet appears to have turned clear, with only the slightest trace left in the middle. I've used these two colours before without any problem and I'm intreged to find out why its happened. Has anyone seen anything like this before? Any ideas would be much appreciated.
Emma
What is your ratio of lt. violet to white compared to previous pot melts, and is it different than you've used in the past?
You will lose some saturation of color, particularly with a transparent and when it is spread thin as it is in a pot melt...That was my first thought anyway, was that you may have ended up with less of a concentration of an already lightly saturated color.
The ratio I used is roughly the same as before, slightly more white. Just checked my notes and the previous work was at temperature for less time, so maybe it was the reduction atmosphere. Other than reducing temps and times needed for pot melts, is there anything I can do to reduce the loss of oxygen? Thanks
Sorry about the delayed reply. The standard way of getting oxygen into a glass melt is by using a nitrate, e.g., sodium or potassium nitrate. Unfortunately, in your case this really isn't feasible because it will both introduce bubbles and change the expansion of your glass.
You could attack the problem from the other direction and try to determine if there was anything in your kiln that caused a reducing atmosphere and caused the color change of the violet glass. Did you use any new frax board or paper in the kiln when you did the melt? The gases given off by the organic binders would cause a reduction atmosphere. Same with anything else organic that burned off in the kiln. So check that first.
The easiest thing to do might be to simply vent the kiln a few times both while heating and once you reach your maximum temperature. This will give you some fresh air inside and **might** help keep the color from changing.
Or pipe a tiny trickle of oxygen in if you have an O2 tank in your shop. However, there are some safety issues involved in going that route.
That's interesting about the violet. I've never had that particular color change at all - even when flameworking. I thought it was pretty stable.
However long periods of time in the kiln can do weird things. Has anyone else experienced color shifts with violet, and I assume we're talking about that rose-tinted purple that is common in all glass types.
Thanks for all your ideas, I've tried another melt with those colours and the same thing happened again. I haven't used anything new in the kiln, so I think its probably the long soak at temp. I'll try venting more but I think I may have to admit defeat. No real loss though, the two melts still look pretty good. Are most violets/purples coloured with manganese? or is it worth trying with other ones? I was thinking of getting a sheet of BE plum.
Yes, most violets/purples are colored with manganese, although some are colored with gold. You can usually tell the gold purples by their higher price.
