In the past, glass props that were readily breakable were made from corn syrup, water and – as the name seems to suggest – sugar. The production of these breakaways worked well for a long time, especially in the case of coloured glassware. On clear glass, however, it left a distinctive yellowish sheen. This did not look good on camera, so another option had to be considered.
This is why break-aways such as bottles and windows are nowadays made of synthetic materials, so that they look real on camera. Another advantage of these synthetic materials is that these break-aways can get wet without melting and can easily be filled with a liquid.
More information on the use of sugar glass breakaways can be found here.