The process of milling is accomplished by passing the dried soap chips through a _soap mill_, which is a machine consisting of usually three or four contiguous, smooth, granite rollers operated by a system of gears and set far enough apart to allow the soap to pass from a hopper to the first roller, from which it is constantly conveyed to each succeeding roller as a thin film, and finally scraped from the last roller to fall into the _milling box_ in thin ribbon form. These mills are often operated in tandem, which necessitates less handling of soap by the operator. The object of milling is to give the soap a glossy, smooth finish and to blend it into a homogeneous mass. The perfume, color, medication or any other material desired are added to the dried soap chips prior to milling. Some manufacturers use an _amalgamator_ to distribute these uniformly through the soap, which eliminates at least one milling. When a white soap is being put through the mill, it is advisable to add from 0.5% to 1% of a good, fine quality of zinc oxide to the soap, if this substance has not been previously added. This serves to remove the yellowish cast and any translucency occasioned by plodding. Too great a quantity of this compound added, later exhibits itself by imparting to the soap a dead white appearance. Inasmuch as the milling process is one upon which the appearance of a finished cake of toilet soap largely depends, it should be carefully done. The number of times a soap should be milled depends upon the character of a soap being worked. It should of course be the object to mill with as high a percentage of moisture as possible. Should the soap become too dry it is advisable to add water directly, rather than wet soap, since water can more easily be distributed through the mass. As a general statement it may be said it is better policy to overmill a soap, rather than not mill it often enough.
After the soap has been thoroughly milled it is ready for plodding. A _plodder_ is so constructed as to take the soap ribbons fed into the hopper by means of a worm screw and continuously force it under great pressure through a jacketed cylinder through which cold water circulates in the rear to compensate the heat produced by friction and hot water at the front, to soften and polish the soap which passes out in solid form in bars of any shape and size depending upon the form of the _shaping plate_ through which it is emitted. The bars run upon a _roller board_, are cut into the required length by a special _cake cutting table_, allowed to dry slightly and pressed either automatically or by a foot power _press_ in any suitable soap _die_. The finished cake is then ready for wrapping and after due time in stock reaches the consumer.