The flash point may prove useful in the evaluation of an essential oil. Unfortunately insufficient data exist to use this property as a criterion of quality for normal, unadulterated oils. However, the flash point has value as an indication of adulteration: additions of adulterants such as alcohol and low boiling mineral spirits will greatly lower the flash point.
Occasionally it is necessary to determine the flash point of a synthetic, solvent, or a mixture because of shipping regulations.
Several types of instruments are available for the determination; e.g., the Pensky-Martin closed tester,50 the Tag closed tester,61 the Cleaveland52 and the Tag open cup testers. The Tag open cup tester is simple, inexpensive, and entirely satisfactory for use in the essential oil industry. The procedure described below is intended primarily for this instrument (see Diagram 4.6).
Procedure: Fill the metal bath with water of about 60o F. (15.6oC.) temperature,54 leaving room for displacement by the glass oil cup which is placed in the water bath. Suspend the thermometer in a vertical position so that the bottom of the bulb is about 1/4 in. from the bottom of the glass cup and so that the back of the glass cup. Fill the glass cup with the oil to be tested in such a manner that the top of the meniscus is exactly at the filling line at room temperature (i.e., 1/4 in. from the upper edge of the cup). Be sure that there is no oil on the outside of the cup or on its upper level edge ; use soft paper to clean the cup in preference to a cloth. Remove any air bubbles from the
50 A description of the instrument and a detailed procedure for its use may be found in A.S.T.M. Designation D93-42.
51 A.S.T.M. Designation D5(>-36.
52 A.S.T.M. Designation D02-33.
53 This procedure is based on the directions for using the Tag Open Fire Tester supplied by the Tagliabue Manufacturing Co., Brooklyn, N. Y.
54 It is customary in the United States to report flash points in degrees Fahrenheit. the thermometer is suspended half way between the center and
Tag open cup tester for the determination of flash point

 DIAGRAM 4.6. Tag open cup tester for the determination of flash point.
surface of the oil. Adjust the horizontal flashing taper guide wire in place. The instrument should stand level and should be protected from drafts. It is desirable that the room be darkened sufficiently so that the flash may be readily discernible.
Avoid breathing over the surface of the oil. Heat the water bath with a small burner so that it will raise the temperature of the oil at a rate not faster than 2o F. (1.1o C.) per min. without removing the burner during the whole operation. Adjust the test flame on the flashing taper so that it is the same size as the metal bead mounted on the instrument. Apply this test flame to the oil at 5o F. (2.8o C.) intervals: hold the flashing taper in a horizontal position and draw it across the guide wire quickly and without pause from left to right. (The time of passage of the test flame across the cup should be approximately 1 sec.) The first or initial flash55 is called the "flash point." Continue heating and testing the oil until the surface ignites and continues to burn until quickly blown out with a mouth-open breath. This burning point temperature is called the "fire test" or "fire point." Repeat the determination and try for a flash at the proper trial temperatures indicated in Table 4.8.
58 The true initial flash should not be confused with a bluish halo that sometimes surround the test flame
(All Temperatures in o F.)

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