[2] From table 51 (refined). [3] From table 54 (grade not specified, but chiefly refined). [4] Production plus imports minus exports. [5] Not available. [6] Preliminary. Up to 1924 (except 1920) imports consisted principally of crude glycerin, much of which was refined in the United States and included in United States production; thereafter imports of refined glycerin became important relative to the crude. Exports are insignificant compared to either production or imports. 17. RAW MATERIALS FOR TAR-ACID RESINS The first tar-acid resins were made from phenol and formaldehyde. As a result this group of resins is frequently spoken of as phenolic resins. This is true despite the fact that coal-tar acids other than phenol, particularly the cresols and xylenols, are used in large volume today. THE TAR ACIDS The term coal-tar acids is applied to certain organic compounds either obtained from or known to be present in coal tar. Probably the best known is phenol or carbolic acid, produced in large quantities in the United States and abroad. Others of commercial importance are ortho, meta, and para cresol and the xylenols. All of these are definite chemical compounds available as such or in mixture with other tar acids. Cresylic acid is a term widely used in commerce for almost any mixture of tar acids. Formerly it was used to designate a mixture of ortho, meta, and para cresols in the proportions in which they are found in coal tar. The higher boiling tar acids (listed in table 56 below the xylenols) have little or no commercial importance at this time. Table 56 lists the tar acids by commercial name, chemical name, boiling point of the pure compound, and average percentage present in coal tar. Boiling point is shown because the several tariff classifications covering tar acids under the acts of 1922 and 1930 (pars. 27 and 1651) depend upon distillation range (boiling points) for classification and assessment of duty (see pp. 119 and 124). TABLE 56.—_Tar acids: Commercial and chemical names, boiling points and average percentage present in coal tar_