Glass particles comprise glass microbubbles. The glass particles have a size distribution with a d50 in the range of from 15 to 100 microns, and have a true density of less than 0.7 g/cm3. The glass particles comprise, on an equivalent weight basis: from 50 to 70 weight percent silica; from 2 to 7 weight percent of boria; from 0.5 to 4 weight percent of weight zinc oxide; from 8 to 17 weight percent of calcia; from 0.8 to 7 weight percent of phosphorus pentoxide; and from 2 to 9 weight percent of sodium oxide.
