Glass is made from sand, and would therefore seem to be in the same Halachic category as earthenware (which is super-absorbent and cannot be kashered). On the other hand, the resultant glass vessel is hard and smooth – unlike earthenware – which would indicate that it is non-absorbent, and would therefore never need to be kashered.
Glass is a difficult material to classify Halachically, because it shares its origin with one class of material, but has physical characteristics that differ from items of that class.
Because of these unusual characteristics, there are a few different opinions in Jewish Law concerning glass dishes:
Rav Yosef Karo rules that you can use glass for both milk and meat, and just rinse them off in between.
Rav Moshe Isserlish writes that glass is like earthenware, and it is therefore forbidden to use the same dishes for both meat and milk.
There is a third opinion that holds that glass is absorbent, but that it can be kashered through the process of “hagala” (immersion in boiling water).
Sephardic Jews rule according to Rav Yosef Karo, and therefore use glass dishes for both meat and milk, while Ashkenazic Jews conduct themselves according to the opinion of Rav Moshe Isserlish, therefore refraining from the use of glass dishes for both. But, because there are different opinions regarding the Halacha, there is room for leniency in cases where extenuating circumstances exist (for instance, a Ba’al Teshuvah who is going for a family visit where the kitchen is not kosher, but glass utensils are used).
As such, glass used as a “kli rishon” is not to be used for both meat and dairy. Turntables in the microwave cannot be used for both meat and dairy.