Dipping-boards, unless freed from adherent glaze by washing after use, create dust whenever ware is placed on, or removed from, them, when they are handled and placed on or taken off the stillage bars, and when they are stacked. Persons gathering at the mangle are exposed to dust if there is any outward current of air from it. The glost-placer raises a slight amount of dust as he takes the ware from the board and places it in the sagger. The dangerous practice formerly almost universal of rubbing the bottoms and rims of cups, etc., either together (without use of an exhaust) or rubbing them on a piece of leather fixed round the chest, is generally replaced by removal of the glaze on a moist piece of flannel, but it is still possible to find men doing it in outlying potteries. In majolica dipping and painting (apart from the obvious risk of splashing and contamination of the hands), danger arises mostly from scraping the edges and under surfaces of the tiles on to which glaze, when applying the background, has overflowed. The amount of glaze so removed is considerable, and if it is not all caught in the trough of water, the floor becomes an added source of danger.
In all the decorative processes—ground-laying, aerographing, colour-dusting, and grinding of colours for aerographing, etc., the danger is one solely arising from dust.
Prevention.
—Meticulous attention to detail, not only in the provision, but also in the maintenance, of the locally-applied exhaust ventilation, alone can allay the danger in the processes to which dust is incidental, such as ware cleaning, gathering at the mangle, glost-placing, and the decorative processes. The Lead Committee considered that, as there was no rapid method of testing the actual degree of moisture, exhaust ventilation might be required in the case of ware that was not cleaned within fifteen minutes of the application of the glaze. Such a requirement would prevent the practice now prevalent of painting as many as three dozen tiles, piling them one on top of another, and then proceeding to the operation of scraping. No danger attaches to removal of glaze with a damp sponge or flannel, but means must always be at hand for washing and damping them. In the dipping-house, (a) impervious floors should be provided, which could be washed down so as to prevent the risks from sweeping, and from glaze drying, and being raised as dust; (b) partial covering of the dipping-tub to prevent splashing and spray; and (c) substitution for the overalls at present worn by persons in the dipping-house, glost-placers, millers and mixers of glaze, majolica paintresses, and others, of overalls of some light waterproof material which could be sponged, or of aprons of waterproof material worn in front of the overalls. Dipping-tubs and walls and floors in close proximity to them can with advantage be painted red. Dipping-boards should be washed with clean water after every time of use. Automatic machines for washing and scrubbing boards are in use in some factories.
To reduce risk or remove the danger of lead poisoning in this industry, use of low solubility glazes or of leadless glazes are advocated. On this point the Lead Committee say: “The effect of melting the lead with silicious matter amounts to imprisoning it in such a manner as to render it less liable to the action of the acids which it meets in passing through the human body, and in consequence largely reduces the likelihood of its absorption into the blood. If the frit is properly compounded, all but a small fraction of the lead is rendered insoluble, and glazes so made are spoken of as ‘low solubility glazes.’ The finished glaze generally contains from 12 to 22 per cent., or more, of lead oxide, but after the process of fritting with sufficient silicious material only from 2 to 5 per cent. remains soluble.”[A]
[A] Raw lead comprises red lead, white lead, and litharge. If introduced in this form as a constituent of glaze it is soluble in dilute acids. If, however, the raw lead is fluxed by heating with a part or the whole of the silica, it is converted into “fritted lead.” The solubility of the frit depends upon the relative proportions of material taken. Thorpe[23], as a result of numerous analyses of lead silicates (after determining their solubility as regards lead), both simple and complex, in use in the potteries and on the Continent, found that the quantity of lead dissolved had no necessary relation to the quantity of lead in the silicate. “Primarily and in the main the insolubility of the lead depends not upon any one oxide or group of oxides, but upon the maintenance of a certain proportion between the whole of the basic oxides on the one hand and the whole of the acidic oxides on the other. If the value of ratio bases/acids is higher than, or approximately equal to, two, the amount of the lead extracted is small, but if it fall much below two, the quantity of lead dissolved begins rapidly to increase.”
On the subject of the use of leadless glazes, the Committee conclude that in all classes of pottery ware a great many articles can be manufactured in a very high state of perfection, with reduction in the cost of production of certain classes of common ware, such as jampots and Persian painted ware; but that in certain other classes, owing to the excessive number of “seconds,” their use would entail increased cost or sacrifice of quality, so much so as to involve loss of important markets; and, finally, that certain kinds of ware, in consequence of difficulties relative to accuracy in reproducing old patterns, colours, or methods of decoration, cannot at present be made at all without use of lead.
In the case of manufacturers who are able to conform to the Thorpe test of low solubility—i.e., glaze which yields to a dilute solution of hydrochloric acid not more than 5 per cent. of its dry weight of a soluble lead compound, calculated as lead monoxide (PbO)—important relaxation of certain special rules are allowed, such as limitation placed on the employment of females and young persons, and periodical medical examination of the workers.
H. R. Rogers[24], one of H.M. Inspectors of Factories, Stoke-on-Trent, has worked out a simple test to show approximately how much lead has been used in the glaze of a piece of pottery. Thus, by treating glazes with hydrofluoric acid for forty seconds, absorbing the liquid with filter paper, precipitating the lead on the paper as the sulphate, dissolving out the sulphate soluble in water, and then precipitating the lead on the paper as sulphide, stains are produced varying, in depth of colour, according to the proportion of lead in the glazes concerned (see [Plate IV.]).
Briefly summarized, the recommendations of the Potteries Committee in regard to the processes are—