JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS lime glasses, are tested, the order of durability may be different for different conditions of test, in this case whether water or acid is used as the attacking medium in a powder test. I)urability tests on glass, as glass, however, must not be confused with durability tests on glass products. Factors other than composition may have some influence on the durability of a product, because of the different uses to which glass is put. (With containers, for example, the effect of surface treatment, heat treatment during manufacture, etc.) This view- point is reflected in the U.S. Pharmocopoeia which classifies glass containers for pharmaceuticals into four types on the basis of durability. Listed in order of decreasing durability, these are-- III Usually soda glass. I Highly resistant glass, usually borosilicate. May be sterilised before or after filling. IV Usually treated soda glass. May be sterilised before or after filling. II Usually treated soda glass. Containers must be sterilised and dried before filling. Container must be sterilised and dried before filling. Type I glasses are ordinarily used for very small containers, and items. like vaccine tubes and syringe cartridges, and a powder test is applied. The other types are applicable to large containers, the test being made on the bottle itself because of the surface area/fluid capacity factor, and the effect of surface treatment. The test is made with distilled water, or one of two. different strengths of sulphuric acid, depending on the nature of the intended contents of the bottles. All tests are carried out in an autoclave at 121øC.. SURFACE 1. "S,dphuring" Long before this process was understood, glassmakers observed the bloom on glassware leaving the old type of direct-fired annealing kilns-- indeed its presence was welcomed as an indication of good annealing. The bloom, which was removed by subsequent washing, consisted of sodium sulphate formed by the reaction of the moist acidic gases (mainly sulphur dioxide) on some of the surface alkali. The silica-rich layer left as a result proved more resistant, at least for a period, to attack by water and in addition was mechanically stronger. I)imbleby • has quoted the following values for the 5 hour extractions by boiling water, from washed widemoutl• jars alike except for annealing atmosphere: Extraction Annealed in absence SO• 2-9 mg Na•O Annealed in presence SO• 0-9 mg

PRODUCTION AND PROPERTIES OF GLASS CONTAINERS 33 In modern annealing lehrs of the muffle type, or electrically heated, sulphur dioxide is often deliberately introduced, either to increase durability or mechanical strength. If the outside only of the containers is to be treated, or if these are of the widemouth type, it is sufficient to burn sulphur in the annealing chamber or introduce sulphur dioxide gas into the lehr. If the insides of the bottles only are to be treated, which is more usually the case, and especially if the bottles are of the narrow necked variety sulphur dioxide is directed individually into the•n from a series of jets dispersed in the annealing chamber, or above the conveyor belt transporting the hot bottles from the forming machine to the annealing lehr. An alternative method is to drop pellets of ammonium sulphate into each bottle immediately before entering the lehr. It must be remembered that the sulphuring action generally proceeds to but a small depth in the glass surface, and that in long term usage the resulting silica-rich layer may be penetrated or removed. 2. Use of Silicones The coating of a glass surface with a water-repellant substance, such as a silicone, appears attractive but it is difficult, as yet, to assess the importance of silicones to the container industry generally, as performance claims differ widely between silicone manufacturer and user. The silicone fihn is not very resistant to alkalis, and therefore silicones are of little use when applied to returnable containers, which undergo chemical cleaning operations. In the pharmaceutical trade, however, silicones fulfil two main purposes. Firstly, they prevent an alteration in the pH value of preparations which are critical in this respect by preventing contact between the preparation and the glass surface, and secondly, they enable the medical practitioner to remove from the container small doses of sometimes very costly materials without losses due to droplet formation on the inside of the container, or drops running down the outside. The silicone, usually a dimethyl poly- siloxane, is applied to the container as an emulsion or solution in organic solvents containing 1-2% by weight of siloxane, by dipping if both surfaces are to be treated, or by a jet of liquid if for the inside only. The excess is allowed to drain off and the container is heated to the temperature required to cure the film, 2 hr at 200øC being a typical figure. This process is said to be easily included in the usual washing and heat sterilising process at little extra cost, and has been carried out for some years by several pharma- ceutical manufacturers. Typical of the somewhat conflicting evidence of the usefulness of silicones in improving the chemical resistance of ordinary containers, are the results of some tests carried out in one of our works on 8 oz square sauce bottles, normally sulphur treated by admission of sulphur dioxide into the annealing lehr. For convenience, the bottles were treated