How is boric acid used to make heat-resistant borosilicate glass?

To produce heat-resistant borosilicate glass, boric acid is first heated to drive off water, forming metaboric acid and eventually boric oxide ($B_2{O}_3$).1

This resulting boric oxide is then mixed with silica to create the glass composition. In this process, boric oxide acts as a flux—a material that lowers the working temperature of the glass melt. By replacing a portion of the sodium oxide and lime typically found in standard soda-lime-silica glass with boric oxide and alumina, manufacturers significantly reduce the glass's coefficient of thermal expansion.123

This reduction in thermal expansion is what makes the final product much less susceptible to thermal shock, allowing it to withstand sudden temperature changes without fracturing. Consequently, this material is highly valued for applications requiring durability under heat, such as laboratory equipment and domestic cookware.31