When the glasses are dry they should be carefully polished with a clean rag. These glasses are used for the first coating and the dried dyed film has to be stripped from them, but unless they are perfectly clean it will not peel, and even then it is not always an easy matter. All sorts of dodges have been suggested, such as collodionizing the glass, waxing it, or polishing with talc; but we may be content with a very simple plan. Add about ten drops of almond or olive oil to 100 ccm of benzol, pour five or six drops of this on the glass and rub over with a tuft of absorbent cotton, and then polish with two pieces of clean dry linen. An old handkerchief again comes in handy, as it is usually fluffless; this will clean the surface and yet leave the merest trace of oil, which makes it easy to strip afterwards.
If the size of the finished filters were to be 5 x 5 cm, we would naturally choose the first glasses 12 x 12 cm, so as to get four filters from each sheet, which gives a chance to pick and choose. To cover 144 qcm at the rate of 700 ccm per square meter, we shall require 10 ccm of dyed gelatine and very small quantities of the dyes, for instance in the case of rose Bengal 0.018 g and 0.028 g tartrazin, which means 1.8 ccm of the first solution and 1.4 ccm of the second. While it is possible to measure these quantities with a good pipette, it is preferable to make at least 50 ccm of the dyed gelatine; we, therefore, measure 34 ccm of the plain gelatine solution, mix 9 ccm of the rose Bengal solution and 7 ccm of the tartrazin, and add these to the gelatine solution; after stirring well, 10 ccm should be coated on the glass.
Every worker has his own particular method of working, and the author is addicted to the use of the pipette for coating, as it has been found easier to lead the gelatine over the surface of the glass with this than by pouring out of a small graduate, and there is less chance of bubbles. If these do occur they can very easily be led to the edge of the glass, where they will do no harm, and can be broken by touching with a spill of blotting or filter paper.
It is important that the room in which the coating is done be not too cold or the gelatine will set before it is possible to spread it over the glass, and the stock of dyed gelatine should be kept at a proper temperature. It is easy to do this with a water bath, and 450 C. (1230 F.) is a comfortable temperature to work at, as the gelatine can then be easily spread, and it sets rather rapidly to an even surface. As soon as thoroughly set, the coated glass should be reared up on edge to dry and the method suggested for drying sensitized plates may be adopted, but dust must be carefully guarded against, and before commencing to coat, the working bench should be well washed down with water so as to ensure that no dust lies about. Rapid drying is not of such moment with filters as with sensitized plates, but it should be as even as possible; if it takes too long, the film may become pitted with small colonies of spots due to bacterial growth, particularly in hot weather.
When the gelatine is perfectly dry, it should be cut all round the edges, about 2 mm inside, with a sharp penknife, and usually it is possible to lift one edge with the knife and, by taking hold of the free edge, to strip the whole film; if not, it should be held for four or five minutes about two feet above a bowl of steaming water, when it should strip without trouble and without stretching. As soon as stripped, the film should be placed between tissue paper and stored between the leaves of a book. The used glasses can be freed from the narrow strip of dyed gelatine by scraping with the knife, re-polished, and again coated with the second lot of dyed gelatine, but it is preferable to have at least six glasses and coat them one after the other, thus making one job of it. Naturally the pipette will be well scoured out with hot water.
It will be seen that the process is not such a difficult one and with a little practice it is easy to become so expert that a failure is rarely met with. But for years the author has given up coating his own filters, except for experimental purposes, and is content to buy commercial filters. The results are absolutely certain and the actual cost is so very little, that taking into consideration the difficulty of getting good glass and the fact that one has to buy much larger quantities of dyes than one needs, commercial filters actually work out cheaper in the end.
Commercial filters can be obtained in the form of film or cemented between glasses. Assuming that the worker has made his own filters or bought commercial film filters, we come to the operation of cementing them, which is not nice. The final glass must be carefully cleaned and the polished surfaces should be placed in contact, pair by pair. We now want to fasten these together like a book cover, and use for this purpose a piece of lantern-slide binding, making quite sure that the edges are even. This can easily be done by placing the wetted strip on a yielding surface, such as an open book; then, holding the two glasses together, lower them on to the strip with an equal margin on each side, press down, and rub the edges of the strip into contact with the sides of the glass. There is no need to let this get absolutely dry, but it should not be wet. The dyed film, cut to the same size as the glass, is slipped between the two glasses and all edges made to coincide. The filter should now be laid down on a sheet of hard card or paper, the top glass and the gelatine film lifted up together, and a pool of balsam poured on the lower glass near the hinge. Enough balsam should be used to cover about one-fourth of the surface of the glass. The gelatine film should then be carefully lowered down on the balsam, another pool of balsam poured on the gelatine and then the top glass lowered. Gentle pressure, starting from the hinge, will force the balsam out to all the edges without the least air bubble. If one does show, it can be chased to the edge by pressure of the fingers. A good sized metal bulldog clip should now be clipped on the glass at right angles to the hinge, as far in as it will go and then another applied opposite the first; the paper hinge should be scraped off the edge with a knife, paying no attention to that on the flat of the glass, and a clip placed here and then one on the fourth side. The filter may then be reared up on one corner to allow the balsam squeezed out from its edges to run down. When all the filters are cemented, begin with the first, wipe off the exuded balsam with filter paper or a bit of old rag, and leave the filters to dry.