The purpose of this Digest is to help a user in selecting a brick by describing the properties that are likely to be significant in relation to the position in which the brick is to be used in the building, for the position largely determines the durability of a brick. All bricks contains some soluble salts, chiefly sulfates of soda, potash, calcium and magnesium. The quantity present, which varies widely, is important because, in sufficient quantities, the salts may lead to efflorescence or sulfate attack of the mortar.