These are actually “macrospheres”, which means that they are large enough to see with the naked eye. We are currently carrying three different sizes and they are excellent for use in portland cement, gypsum cement, and all of the various resins we carry. The great thing about these critters is that they are “green”, meaning that they are made from recycled glass. Use as you would use any aggregate in cement and make lighter castings. Of course you must realize that strength will be diminished proportionate to the amount of the glass spheres you use. The small size is about the size of buckshot, medium is about like BB’s and the large is about like small pea gravel.