Char also has a great surface attraction, which causes it to collect particles of coloring matter that may be present, and thus acts as an excellent filtering agent. New char should be thoroughly washed with pure water until all the impurities are removed. With the end in view of determining when the last traces of chlorine have disappeared, chemical tests are made on the wash waters. Nitric acid and silver nitrate are employed for this purpose. After animal char has been used for some time in the filters and fails to do its work efficiently, it is reburned, or revived, as it is called. Ordinarily the best results are obtained after a char has been used several times.
Reburning of the char at too high a temperature should be avoided, as it incurs an unnecessary loss of fuel, besides causing serious injury to the char by a contraction of the pores. Since, as stated previously, the main value of the char as a clarifying and filtering medium lies in the fact of its porosity, anything which reduces this will greatly impair its efficiency. One thing in connection with the bone-char process of making white sugars is that it is expensive and should not be attempted except on a large scale, since the initial expense of installation, as well as the cost of running, is very great. The writer is sometimes asked by managers of small factories, turning out plantation yellow clarified sugars, if it would not pay them to employ bone-char filters to use in connection with the remainder of their factory, in order to be able to work up an industry with the low-grade open-kettle sugars, during the intercampaign. Most assuredly such a combination of small plantation factory and refinery would not be a paying affair. It takes men of experience and special training to carry out successfully the more detailed work in any technical line. One thing, however, can be very successfully done by these factories, and that is to make a first-class plantation white sugar which will command a ready price in the local markets, or even suffice for export, if the proper manufacturing methods are used.
It is not presumed that any one planter will use all of the clarifying reagents mentioned above, but he should choose the ones to fit his individual needs, and secure his supply early, since a great deal of time is required to transport supplies from the place of manufacture to these Islands. This is especially the case when the place of manufacture happens to be in Europe, as is true with a number of the patented clarifying reagents.