Scientific American Supplement, No. 821 | Page 9

parts raw paper, 176.4 partially
distilled tar. Experiments on a larger as well as a smaller scale reduced

this quantity to an average of 141.5 parts for one hundred parts raw
paper. The weight of sanded paper is very variable, as it depends
altogether upon the size of the sand grains. It may be stated generally
that the weight of the sand is as large as that of the tarred paper.
The kinds of roofing paper saturated with other additions besides coal
tar form a separate class, in order to neutralize the defects inherent in
coal tar. These additions were originally for the purpose of thickening
the paper and making it stiffer and drier. The most ordinary and
cheapest thickener was the coal pitch. Although the resinous substances
are increased thereby, still the light tar oils remain to evaporate, and the
paper prepared with such a substance readily becomes hard and brittle.
A better addition is the natural asphaltum, because it resists better the
destroying influence of the decomposition process, and also, to a
certain degree, protects the coal tar in which it is dissolved. The
addition of natural asphaltum doubtless caused the name of "asphaltum
roofing paper." Resin, sulphur, wood tar and other substances were also
used as additions; each manufacturer kept his method secret, however,
and simply pointed out by high sounding title in what manner his paper
was composed. In most cases, however, this appellation was applied to
the ordinary tar paper; the impregnating substance was mixed only with
coal pitch, or else a roofing paper saturated with distilled tar. The costly
additions, by the use of which a high grade of roofing paper can
doubtless be produced, largely increased its price, and on account of the
constant fall of prices of the article, its use became rather one of those
things "more honored in the breach than in the observance," and was
dispensed with whenever practicable. The crude paper is the foundation
of the roofing paper. The qualities of a good, unadulterated paper have
already been stated. At times, the crude paper contains too many earthy
ingredients which impair the cohesion of the felted fibrous substance,
and which especially the carbonate of lime is very injurious, as it
readily effects the decomposition of the coal tar. The percentage of
wool, upon which the durability of the paper depends very largely, is
very small in some of the paper found in the market. In place of woolen
rags, cheap substitutes have been used, such as waste, which contains
vegetable fibers. Since this cannot resist the decomposition process for
any length of time, it is evident that the roofing paper which contains a
noticeable quantity of vegetable fibers cannot be very durable. To judge

from the endeavors made to improve the coal tar, it may be concluded
that this material does not fully comply with its function of making the
roofing paper perfectly and durably waterproof. The coal tar, be it
either crude or distilled, is not a perfect impregnating material, and the
roofing paper, saturated with it, possesses several defects. Let us in the
following try to ascertain their shortcomings, and then express our idea
in what manner the roofing paper may be improved. It was previously
mentioned that every tar roofing paper will, after a greater or smaller
lapse of time, assume a dry, porous, friable condition, caused by the
volatilization of a part of the constituents of the tar. This alteration is
materially assisted by the oxygen of the air, which causes the latter to
become resinous and exerts a chemical influence upon them. By the
volatilization of the lighter tar oils, pores are generated between the
fibers of the roofing paper, into which the air and humidity penetrate.
In consequence of the greatly enlarged surface, not only the solid
ingredients of the tar, which still remain unaltered, are exposed to the
action of the oxygen, but also the fibers of the roofing paper are
exposed to decomposition. How destructive the alternating influence of
the oxygen and the atmospheric precipitations are for the roofing paper
will be shown by the following results of tests. It will have been
observed that the rain water running from an old paper roof, especially
after dry weather, has a yellowish, sometimes a brown yellow color.
The supposition that this colored rain water might contain
decomposition products of the roofing paper readily prompted itself,
and it has been collected and analyzed at different seasons of the year.
After a period of several weeks of fair weather during the summer, rain
fell, and the sample of water running from a roof was caught and
evaporated; the residue when dried weighed 1.68 grammes. It was of a
brownish black color, fusible in heat and readily soluble, with a yellow
brown color in water. The