TRANSACTIONS OP SECTION C. 40l 



2. Some Geological Characters of Sands used in Glass Manufacttire. 

 By P. G. H. BoswELL, D.Sc, F.G.S. 



At a time when it is necessary to know the extent and value of our national 

 resources of sands suitable for various industrial purposes, including glass- 

 manufacture, it is especially desirable that we should realise the particular 

 properties of such sands and the geological conditions under which the deposits 

 occur in the field. 



(a) In chemical composition, for all general purposes of glass-making, the 

 sand should contain a very high proportion of silica (SiO,), if possible, over 

 99 per cent. The percentage of iron (estimated as Fe.Oj) should be as low as 

 possible. For optical glass, table-ware (' crystal'), &c., it should not rise above 

 0-05 per cent. ; for laboratory-ware, globes, and all second-grade glass-wai-e, a 

 percentage up to 0'08 is permissible ; for plate- and window-glass and good white 

 bottle-glass the proportion may reach O'l or 0"2 per cent. ; and for rough 

 bottle-glass and other similar work a limit of 2 per cent, may be admitted. For 

 refractory glass, such as that used for thermometers, gauges, certain laboratory- 

 ware, &c., it is an advantage to find a sand bearing 4 per cent, or more alumina. 

 Unfortunately, most British sands bearing alumina carry also iron and other 

 undesirable impurities. Other bases, such, as lime, magnesia, titanium, and 

 alkalies, should, if present at all, exist only in negligible quantities. In the 

 analyses the loss on ignition should also appear; it yields an indication of the 

 amount of water and organic substances present. The latter are not objectionable 

 as they usually ' burn out.' 



The analysis of one of the best British glass-sands, a sample of Lower 

 Greensand from Aylesbury, indicates : SiO,, 99-80 per cent. ; AUOj, 0-32 per 

 c^ent. ; Fefi,, 0'03 per cent. ; loss on ignition, 022 per cent. ; total, 100-37 per 

 cent. With this may be compared a well-known German glass-sand from Lippe : 

 SiO,, 99-88 per cent. ; AUO,, O'lS per cent. ; Fefi^, 0-02 per cent, j loss on 

 ignition, 0-21 per cent. ; total, 100-29 per cent. 



(6) For all but the highest-quality glass, where the cost of crushing the raw 

 material to a fine even state, with suitable subsequent treatment, is not prohibi- 

 tive, the mechanical composition is of the utmost importance. The sand used 

 should, if possible, be perfectly graded : that is, it should be composed of grains 

 all of the same size. Such perfection of grading is not attained as a result of 

 natural agencies ; the best-graded natural deposits contain over 90 per cent, of 

 grains of one grade, which, for glass-making purposes, is preferably the medium- 

 sand grade (diameter > i and < ^ mm.). A high percentage of the fine-sand 

 grade (diameter > i and < ^ mm.) would be even more preferable, but 

 suitable sands with a high proportion of this grade are not of common occurrence 

 in this country. Coarser sand-grains are not desirable, and, if present, should 

 be removed by sieving. Very fine sand, silt, and clay-grades are inimical, and 

 must be removed by washing. As examples of well-graded glass-sands may be 

 mentioned: — Dutch sand, >J and <1 mm. diameter, 0-4 per cent.; >J and < ^, 

 94-4 per cent.; >^ and <i, 5-1 per cent.; >^ and <^, O'l per cent?; 

 <T^ mm., 0-0 per cent. ; total sand-grade, >^ mm. diameter, 99-9 per cent 

 Kmg's Lynn (Lower Greensand), >J and <1 mm., O-Q per cent.; >J and < J, 

 94-8 per cent.; >^ and < J, 4-9 per cent.; >^ and < ^, 0-2 per cent.; 

 <T^ min-' O'l per cent. ; total sand-grade, >^ mm. diameter, 99-7 per cent. 



(c) The mineral composition should be as simple as possible. Briefly put, 

 the sand should as far as possible contain only quartz, or quartz and felspar, 

 and the heavy detrital minerals present should 'be small in quantity and simple 

 in composition. 



The treatment of sands (whether chemical, to remove iron, or mechanical, 

 to ensure good grading) often involves prohibitive expense. It is therefore of 

 considerable importance, as well as of some interest, to look into the geological 

 conditioni? under which desirable glass-sands occur. We may thus receive clues 

 to the existence of further supplies by knowing the kind of deposits in which 

 they are met, and the special conditions under which we may expect to find 

 them.^ The important supplies of glass-sands occurring in Western Europe are 

 associated with organic matter of planty origin. In support of this statement 

 we may enumerate : Lippe sand, associated with rafts of braunkohle, in beds 

 1916 



