bj
Your 1650 temperature, which I presume is fahrenheit, is equivalent to about 900 degree centigrade, but I am using 930 to melt my glass. Brad Walker's book "Contemporary Warm Glass" says that curved cracks through the middle of the piece are caused by improper annealing. What temperature are you opening the kiln at? I have had some cracks in the past where I became impatient to find out what pattern was in the melt and opened the kiln door too early, say about 80 deg. cent. As I liked the pattern in the piece, I kept it on the shelf, cut a border for it and covered the whole with clear fusible. The cracks healed themselves and the ghost of them hardly showed up in the final piece.
I haven't heard of duraboard. Presumably it is a thicker type of fibre paper or if not, does it require kilnwash just like the shelves. I have used fibre paper for some early melts but did not like the finish on the reverse of the melt so went back to Hotline shelf primer. Another good reason is that it is a lot cheaper than the fibre paper. I have tried boron nitride which is a lot more expensive, but I found that it gave no better finish than the hotline primer at high temperatures and still needed to be cleaned off the shelf after each melt.
Temperatures
bj
Your 1650 temperature, which I presume is fahrenheit, is equivalent to about 900 degree centigrade, but I am using 930 to melt my glass. Brad Walker's book "Contemporary Warm Glass" says that curved cracks through the middle of the piece are caused by improper annealing. What temperature are you opening the kiln at? I have had some cracks in the past where I became impatient to find out what pattern was in the melt and opened the kiln door too early, say about 80 deg. cent. As I liked the pattern in the piece, I kept it on the shelf, cut a border for it and covered the whole with clear fusible. The cracks healed themselves and the ghost of them hardly showed up in the final piece.
I haven't heard of duraboard. Presumably it is a thicker type of fibre paper or if not, does it require kilnwash just like the shelves. I have used fibre paper for some early melts but did not like the finish on the reverse of the melt so went back to Hotline shelf primer. Another good reason is that it is a lot cheaper than the fibre paper. I have tried boron nitride which is a lot more expensive, but I found that it gave no better finish than the hotline primer at high temperatures and still needed to be cleaned off the shelf after each melt.