We have decided that there are two types of wine bottles (and glass jars). One type has a flat bottom while the other has a conical bottom. When slumping a plain, non painted bottle or jar, it really doesn't matter how the bottom looks when finished. When slumping a painted bottle or jar with a flat bottom, more care must be taken so as not to allow the bottom to completely fold over the painted area, thus obscuring part of the image. To do this, a bottle or jar should be only partially slumped, so it must be watched, sometimes minute by minute, at the very end of the kiln firing process to make sure that the slump is stopped before the bottom of the bottle folds over completely. Another way to completely melt a bottle or jar with a flat bottom is to lay the bottle with the front side laying on the kiln shelf (with shelf paper underneath). Once melted flat, the bottom of the bottle will be melted flat onto the back of the bottle. Unfortunately the front side of the bottle will not not have a glossy finish if done this way. So if a glossy finish is wanted on the front of the bottle, it will have to be re-fired with the front side of the bottle facing up, this time. We have found that when firing a bottle or jar for a second time in our kilns, we must slow down our firing schedule or the bottle will break. The schedule not only needs to be slowed down during the heating process, but also during the cooling process as well.
Bottles with a conical bottom are quite easy to melt since when completely melted, the cone shape easily flattens out inside of the bottle so it is not visible on the outside of the bottle. These bottles work great in wine bottle molds and melted flat on a kiln shelf.
We advise glass artists to always use shelf paper. We like Bullseye thin fire shelf paper, it doesn't have a texture that can mar a glass surface. It is easy for a bottle to end up with permanent kiln wash marks or stains when a bottle is laid on a kiln shelf or in a mold without any shelf paper as a buffer.
It is also important to use some sort of devit spray on bottles and jars. Although some artists seem to like the look of devitrification, most don't. We like to use the borax soap mixture on our bottles and jars. It really works and it's easy to make, click on this link to go to our Borax devitrification spray information page!
Blue and clear bottles and jars seem to get devitrification easier than green and brown bottles. For that reason, if you don't want devitrification, you have to speed up the time on your kiln schedule from 1300 degrees to the hottest temperature desired. The less amount of time spent in the that temperature range, the less chance of getting devitrification on your glass.