WHY DOES MY GLASS SCORCH AND WHAT CAN I DO ABOUT IT?

First, glasses such as reds, yellows, oranges, lime greens, etc. seem to scorch easily.  When you insert metal into the glass it actually concentrates the heat even more.  You often find the kiln shelf under a bail, and the glass around it, turning anything from gray to dark black.  Here are the results of some quick experimentation (four days) that I did to try to see how to solve the problem..

THE EXPERIMENTS

Fuse1     Product: An orange Bullseye glass with ripple dichroic fused onto it.

Fuse2    Desire: To make pieces (cabochons) for jewelry.

Fuse3    Problem:  The bottom of the glass scorches.
This is the back of the glass cut from the sheet.  Notice the very dark, scorch marks, like "hot spot" occurred in certain places.  My 18" kiln was used and fired at 800 deg/hr to 1450 deg. F. and then held for 10 minutes.  The glass was placed on thin fire shelf paper on a kiln shelf that had been covered with kiln wash.

Ceramic kiln shelves take a lot of heat and hold it well.  I'm going to try to insulte the shelf from the glass and see if that helps.


Fuse4   Glass fired on 1/32 fiber paper.
The black is gone,  The insulation worked.  Only now there is another problem.  If you look close you can see that there is much more texture to the back.  If this is O.K., then this is a great solution. I wasn't happy and wanted a smoother finish.  Let's try several pieces of thin fire paper.


Fuse5  Two sheets of thin fire under the glass. Nope!. Less but still there. What else to try?
Fuse6                                             Fuse7
This is with 3 thin fire papers.                              This is a thin fire paper over the 1/32" fiber paper.

Both still show some darkening although the one on the right is better.

Since the 1/32" by itself didn't show scorch mark, but the 1/32" with the thin fire on top did,  I'm confused.  Does the thin fire sheet itself cause the problem?   Maybe.  Let's try something new.

I'm going to make a 1/2" fiber board base, harden it, sand it smooth and set it on the ceramic shelf.  Maybe this will work?

Fuse8 The result of using the fiber board.

There are no scorch marks.  There is a little texture, but not near as much as when using the 1/32" fiber paper.  This is acceptable to me and I can use it for difficult colors.

Let's try different colors, which usually scorch.  (Notice the fiber board.)

Fuse12           Fuse13

          Front side of glass.                                 Back side of glass.

This is totally acceptable.  The glass is clean and as vibrant as it was before firing.  And no kiln wash sticks to the glass.  When making the fiber board, soak it well in hardner.  Let it dry outside in the sun for several hours and then fire it to 1100 deg. F. in a kiln.  It smells horrible.   I leave my kiln door open during firing, put on a fan, a evacuate the room.   When its done, put on a couple coats of kiln wash. The board should do for many, many firings.

ANOTHER POSSIBLE SOLUTION
 The insulation prevents a "hot spot" which occurs when some glass is used directly on ceramic kiln shelves.    I wonder if reducing the heat would work. Remember that fuse rates are a function of
both temperature and time.  

Let's decrease the temperature and lengthen the time. Instead of firing to 1450 deg. F and holding for 10 minutes, I'll try firing to 1370 deg. F. and holding for 40 minutes.  

Fuse9 This time not blobs, but even graying.
I checked the kiln shelf, and it shows no similar pattern.  So I know its not coming from anything on the shelf (which has a coating of kiln wash on it.)

Fuse10 The top of the last fuse.
With the lower temperature and longer time, the top was not quite smooth.  It was not a full fuse. Let's try 1350 deg. F. for 2 hours and see what happens.

Fuse11  Much better, with very little clouding.  But was it smooth?
The piece did come out smooth on top and this may be an acceptable solution if you have the time. In fact, a slightly lower temperature and longer hold might produce better results.

Fuse14       Fuse15  How about with metal bails.

Well, the front and back didn't show any scorching, but as you can see from the picture on the right, there is a little darkening around the metal.  So you may still get some discoloration around the bail, but it is much small when fused on fiber board.  Here is the worst case piece of several I fired with bails.  As you can see it is minimal.  If your need perfection, none of these techniques work well with metal insert.  Ces't la vie.  Sometimes life isn't perfect.

What we have found is that it is very difficult to remove darkening from certain colors.  The clearest piece was that fused on 1/32" fiber paper, however, it left some texture. Good color and less texture was found using the sanded 12" fiber board. (Make sure you harden it so you can reuse it.  Hope this helps.

ANOTHER THING YOU LEARNED:
There are often problems that occur when we work.  Like bubbles in glass, there are usually ways to reduce them, but maybe not totally eliminate them. (at least within our cost range).

All artisans need to realize that a "failure" is not a "failure" unless you allow it to be.  Instead of focusing on the "failure" focus on the opportunity to learn something new.  What would your work be worth if it wasn't so hard to learn?  Where would the skill be?