_Perfume._--The materials used in perfuming soap will be dealt with fully in the next chapter. The quantity necessary to be added varies considerably with the nature of the essential oils, and also the price at which the soap is intended to be sold. In the cheaper grades of milled soaps the quantity will range from 10-30 fluid ozs. per cwt., and but rarely exceeds 18-20 ozs., whereas in more costly soaps as much as 40-50 fluid ozs. are sometimes added to the cwt. _Colouring Matter._--During recent years an outcry has been made against highly coloured soaps, and the highest class soaps have been manufactured either colourless or at the most with only a very delicate tint. It is obvious that a white soap guarantees the use of only the highest grade oils and fats, and excludes the introduction of any rosin, and, so far, the desire for a white soap is doubtless justified. Many perfumes, however, tend to quickly discolour a soap, hence the advantage of giving it a slight tint. For this purpose a vegetable colouring matter is preferable, and chlorophyll is very suitable. [Illustration: FIG. 23.--Compressor.] A demand still exists for brightly coloured soaps, and this is usually met by the use of coal-tar dyes. The quantity required is of course extremely small, so that no harm or disagreeable result could possibly arise from their use. _Neutralising and Superfatting Material._--If desired, the final neutralisation of free alkali can be carried out during the milling process, any superfatting material being added at the same time. The chief neutralising reagents have already been mentioned in Chapter VI. With regard to superfatting material, the quantity of this should be very small, not exceeding 6-8 ozs. per cwt: The most suitable materials are vaseline, lanoline, or spermaceti. [Illustration: FIG. 24--Hand soap-stamping press.] (iii.) _Compressing._--The next stage is the compression and binding of the soap ribbons into a solid bar suitable for stamping, and the plant used (Fig. 23) for this purpose is substantially the same in all factories. The soap is fed through a hopper into a strong metal conical-shaped tube like a cannon, which tapers towards the nozzle, and in which a single or twin screw is moving, and the soap is thereby forced through a perforated metallic disc, subjected to great pressure, and compressed. The screws must be kept uniformly covered with shavings during compression to obviate air bubbles in the soap. [Illustration: FIG. 25.--Screw press.]