Boro 3.3 or Borosilicate 3.3 is a type of glass having very low coefficient of thermal expansion Borosilicate glass 3.3 is highly resistant to water effects, neutral and acid solutions, heavy acids and their mixtures, to chlorine, bromine, iodine and organic compounds. Even in long-term effects and at temperatures above 100 °C, this glass outstrips with its chemical durability most metals and other raw materials. Class of Resistance to Acid Effects (ISO 1776 a DIN 12116) is Class S1.