Having looked at the harmonised commodity description notes, this type of glass is classified under CN 70031 or CN 7004. Note 5 to this chapter provides that the expression "glass" includes fused quartz and other fused silica. Note 5 also states "the chapter also covers, glass having a low coefficient of expansion for example borosilicate glass". Therefore there isn't a specific dedicated Prodcom commodity code just for this type of glass. It would therefore be included depending on manufacturing process under one of the following codes along with other similar types of glass: 23111110- flat glass, Non-wired sheets of cast or rolled glass, whether or not with absorbent, reflecting or non-reflecting layer, but not otherwise worked INCLUDING: - optical 23111150- flat glass, Sheets of drawn glass or blown glass, whether or not with absorbent, reflecting or non-reflecting layer, but not otherwise worked.