Each one from my array of slides has its hallmark tone. The ceramic is subtly different from the porcelain. Maybe it has something to do with the touch or grip on the string. They have a certain amount of 'drag' as opposed to glass. I have fooled around with a Coracidin bottle, but like other glass slides, it doesn't 'hug' the strings like the others I prefer -- such as those mentioned above and even the fibre-optic.
I even like the PVC ones that my friend, Mark has given me. They have a certain bite and short decay presumably similar to a hambone.
Copper is good too!
While recording a CD in January 2010, I stuck mainly to a small brass flared slide and a ceramic 'Moonshine' slide for those numbers which I was singing and playing live with the band, and I used a 'Chrome Dome' for the instrumentals where a stronger 'presence' was needed for the thicker bass strings.
Stay tuned for some updates on my slides.