=Sodium Nitrate=, or =Chili Nitre=, is the corresponding soda salt to potash nitre. It is much cheaper, but less pure; it has a similar but not nearly so powerful an oxidising action in the glass as potash nitre. It is exported from Chili, where it exists naturally in a crude state as “Caliche,” from which the nitrate is refined by recrystallisation. =Boric Acid= acts as an acid in glass, as does silicic acid. It renders glass more fusible and brilliant; it has a searching action upon the colourising properties of certain metallic oxides when they are dissolved in the glass. It is an expensive ingredient, but is considerably used in optical and special chemical glassware in replacing a portion of the silicates ordinarily used and forming borates. It cannot be used in large amounts, as an excess produces glass of a less stable nature. =Borax=, or =Borate of Soda=, consists of boric acid combined with soda. It is a very useful glassmaking material and is an active fluxing agent. If used in excess in glass mixtures it causes considerable ebullition, or boiling of the metal. In moderate proportions it is used in the manufacture of enamels for glass, as it helps to dissolve the colorific oxides and diffuse the colouring throughout the enamel mass. =Tincal=, and =Borate of Lime=, are other forms in which borates may be introduced into glass. =Carbonate of Lime=, =Limespar=, =Limestone=, =Paris White=, or =Whitening= are all forms of =Calcium Carbonate=. It is an earthy base and is added to the simple alkaline silicates and borates to form insoluble combinations or double silicates of soda and lime. By the use of lime, glasses are rendered more permanent and unchangeable when in use. Lime forms a very powerful flux at high temperatures. The quantity used must be carefully regulated according to the proportion of other bases present; otherwise an inferior or less stable glass may be produced. In excess it causes glass to assume a devitrified state. =Dolomite= is a _Magnesium Limestone_, and is a natural stone which is available for use in making glass in tank furnaces. =Fluorspar=, or =Fluoride of Lime=, is used in giving opacity and translucency to glass. It can only be used in small amounts, as the presence of any large proportion attacks the clay of the pots, causing serious damage by the sharp cutting chemical action due to the evolution of fluorine gas. =Phosphate of Lime= is another material which produces opacity and translucency, but does not seriously attack the pots. Bone ash is a form of phosphate of lime, and is procured by calcining bones until all organic matter is consumed.