In Germany the architect Gilly was first to become interested in tar paper roofing, and recommended it in his architecture for the country. Nevertheless the new style of roof covering was but little employed, and was finally abandoned during the first year of the 19th century. It was revived again in 1840, when people began to take a renewed interest in tar paper roofs, the method of manufacturing an impermeable paper being already so far perfected that the squares of paper were dipped in tar until thoroughly saturated. The roof constructed of these waterproof paper sheets proved itself to be a durable covering, being unimpenetrable to atmospheric precipitations, and soon several factories commenced manufacturing the paper. The product was improved continually and its method of manufacture perfected. The good qualities of tar paper roofs being recognized by the public, they were gradually adopted. The costly pine tar was soon replaced by the cheaper coal tar. Square sheets of paper were made at first; they were dipped sufficiently long in ordinary heated coal tar, until perfectly saturated. The excess of tar was then permitted to drip off, and the sheets were dried in the air. The improvement of passing them through rollers to get rid of the surplus tar was reserved for a future time, when an enterprising manufacturer commenced to make endless tar paper in place of sheets. Special apparatus were constructed to impregnate these rolls with tar; they were imperfect at first, but gradually improved to a high degree. Much progress was also made in the construction of the roofs, and several methods of covering were devised. The defects caused by the old method of nailing the tar paper direct upon the roof boarding were corrected; the consequence of this method was that the paper was apt to tear, caused by the unequal expansion of the roofing boards and paper, and this soon led to the idea of making the latter independent of the former by nailing the sides of the paper upon strips running parallel with the gable. The use of endless tar paper proved to be an essential advantage, because the number of seams as well as places where it had to be nailed to the roof boarding was largely decreased. The manufacture of tar paper has remained at about the same stage and no essential improvements have been made up to the present. As partial improvement may be mentioned the preparation of tar, especially since the introduction of the tar distillery, and the manufacture of special roof lacquers, which have been used for coating in place of the coal tar. As an essential progress in the tar paper roofing may be mentioned the invention of the double tar paper roof, and the wood cement roof, which is regarded as an offshoot.
The tar paper industry has, within the last forty years, assumed great dimensions, and the preferences for this roofing are gaining ground daily. In view of the small weight of the covering material, the wood construction of the roof can be much lighter, and the building is therefore less strained by the weight of the roof than one with the other kind, so that the outer walls need not be as heavy. Considering the price, the paper roof is not only cheaper than other fireproof roofs, but its light weight makes it possible for the whole building to be constructed lighter and cheaper. The durability of the tar paper roof is satisfactory, if carefully made of good material; the double tar paper roof, the gravel double roof, and the wood cement roof are distinguished by their great durability.
These roofs may be used for all kinds of buildings, and not only are factories, storehouses, and country buildings covered with it, but also many dwellings. The most stylish residences and villas are at present being inclosed with the more durable kinds; the double roof, the gravel double roof, and the wood cement roof. For factory buildings, which are constantly shaken by the vibrations of the machinery, the tar paper roof is preferable to any other.
In order to ascertain to what degree tar paper roofs would resist fire, experiments were instituted at the instigation of some of the larger manufacturers of roofing paper, in the presence of experts, architects, and others, embracing the most severe tests, and it was fully proved that the tar paper roof is as fireproof as any other. These experiments were made in two different ways; first, the readiness of ignition of the tar paper roof by a spark or flame from the outside was considered, and, second, it was tested in how far it would resist a fire in the interior of the building. In the former case, it was ascertained that a bright, intense fire could be kept burning upon the roof for some time, without igniting the woodwork of the roof, but heat from above caused some of the more volatile constituents of the tar to be expelled, whereby small flames appeared upon the surface within the limits of the fire; the roofing paper was not completely destroyed. There always remained a cohesive substance, although it was charred and friable, which by reason of its bad conductivity of heat protected the roof boarding to such an extent that it was "browned" only by the developed tar vapors. A fire was next started within a building covered with a tar paper roof; the flame touched the roof boarding, which partly commenced to char and smoulder, but the bright burning of the wood was prevented by the air-tight condition of the roof; the fire gases could not escape from the building. The smoke collecting under the roof prevented the entrance of fresh air, in consequence of which the want of oxygen smothered the fire. The roofing paper remained unchanged. By making openings in the sides of the building so that the fire gases could escape, the wood part of the roof was consumed, but the roofing paper itself was only charred and did not burn. After removing the fire in contact with the paper, this ceased burning at once and evinced no disposition whatever to spread. In large conflagrations, also, the tar paper roofs behaved in identically a similar manner. Many instances have occurred where the tar paper roof prevented the fire from spreading inside the building, and developing with sufficient intensity to work injury.
As it is of interest to the roofer to know the manner of making the material he uses, we give in the following a short description of the manufacture of roofing paper. At first, when square sheets were used exclusively, the raw paper consisted of ordinary dipped or formed sheets. The materials used in its manufacture were common woolen rags and other material. In order to prepare the pulp from the rags it is necessary to cut them so small that the fabric is entirely dissolved and converted into short fibers. The rags are for this purpose first cut into pieces, which are again reduced by special machines. The rags are cut in a rag cutting machine, which was formerly constructed similar to a feed cutter; later on, more complicated machines of various constructions were employed. It is not our task to describe the various kinds, but we remain content with the general remark that they are all based on the principles of causing revolving knives to operate upon the rags. The careful cleansing of the cut rags, necessary for the manufacture of paper, is not required for roofing paper. It is sufficient to rinse away the sand and other solid extraneous matter. The further reduction of the cut rags was formerly performed in a stamp mill, which is no longer employed, the pulp mill or rag engine being universally used.
The construction of this engine may be described as follows: A box or trough of wood, iron, or stone is by a partition divided into two parts which are connected at their ends. At one side upon the bottom of the box lies an oakwood block, called the back fall. In a hollow of this back fall is sunk the so-called plate, furnished with a number of sharp steel cutters or knives, lying alongside of each other. A roller of solid oakwood, the circumference of which is also furnished with sharp steel cutters or knives, is fastened upon a shaft and revolves within the hollow. The journal bearings of the shaft are let into and fastened in movable wooden carriers. The carriers of the bearings may be raised and lowered by turning suitable thumbscrews, whereby the distance between the roller and the back fall is increased or decreased. The whole is above covered with a dome, the so-called case, to prevent the throwing out of the mass under the operation of grinding. The roller is revolved with a velocity of from 100 to 150 revolutions per minute, whereby the rags are sucked in between the roller and the back fall and cut and torn between the knives. At the beginning of the operation, the distance between the roller and the back fall is made as great as possible, the intention being less to cut the rags than to wash them thoroughly. The dirty water is then drawn off and replaced by clean, and the space of the grinding apparatus is lessened gradually, so as to cut the rags between the knives. The mass is constantly kept in motion and each piece of rag passes repeatedly between the knives. The case protects the mass from being thrown out by the centrifugal force. The work of beating the rags is ended in a few hours, and the ensuing thin paste is drawn off into the pulp chest, this being a square box lined with lead.
From the pulp chest it passes to the form of the paper machine. This form consists of an endless fine web of brass wire, which revolves around rollers. The upper part of this form rests upon a number of hollow copper rollers, whereby a level place is formed. The form revolves uniformly around the two end rollers, and has at the same time a vibratory motion, by which the pulp running upon the form is spread out uniformly and conducted along, more flowing on as the latter progresses. The water escapes rapidly through the close wire web. In order to limit the form on the sides two endless leather straps revolve around the rollers on each side, which touch with their lower parts the form on both sides and confine the fluid within a proper breadth. The thickness of the pulp is regulated at the head of the form by a brass rule standing at a certain height; its function is to level the pulp and distribute it at a certain thickness. The continually moving pulp layer assumes greater consistency the nearer it approaches to the dandy roll. This is a cylinder covered with brass wire, and is for the purpose of compressing the paper, after it has left the form, and free it from a great part of the water, which escapes into a box. The paper is now freed of a good deal of the fluid, and assumes a consistency with which it is enabled to leave the form, which now commences to return underneath the paper, passing on to an endless felt, which revolves around rollers and delivers it to two iron rolls. The paper passes through a second pair of iron rollers, the interiors of which are heated by steam. These rollers cause the last of the water to be evaporated, so that it can then be rolled upon reels. A special arrangement shaves the edges to the exact size required.
The paper is made in different thicknesses and designated by numbers to the size and weight.
Waste paper, bookbinders' shavings, etc., can be used for making the paper. As much wool as possible should be employed, because the wool fiber has a greater resistance than vegetable fiber to the effects of the temperature. By wool fiber is understood the horny substance resembling hair, with the difference that the former has no marrowy tissue. The covering pellicle of the wool fiber consists of flat, mostly elongated leaves, with more or less corners, lying over each other like scales, which makes the surface of the fiber rough; this condition, together with the inclination of curling, renders it capable of felting readily. Pure wool consists of a horny substance, containing both nitrogen and sulphur, and dissolves in a potash solution. In a clean condition, the wool contains from 0.3 to 0.5 per cent. of ash. It is very hygroscopical, and under ordinary circumstances it contains from 13 to 16 per cent. humidity, in dry air from 7 to 11 per cent., which can be entirely expelled at a temperature of from 226 to 230 degrees Fahrenheit. Wool when ignited does not burn with a bright flame, as vegetable fiber does, but consumes with a feeble smouldering glow, soon extinguishes, spreading a disagreeable pungent vapor, as of burning horn. By placing a test tube with a solution of five parts caustic potash in 100 parts water, a mixture of vegetable fibers and wool fibers, the latter dissolve if the fluid is brought to boiling above an alcohol flame, while the cotton and linen fibers remain intact.
The solubility of the woolen fibers in potash lye is a ready means of ascertaining the percentage of wool fiber in the paper. An exhaustive analysis of the latter can be performed in the following manner: A known quantity of the paper is slowly dried in a drying apparatus at temperature of 230° Fahrenheit, until a sample weighed on a scale remains constant. The loss of weight indicates the degree of humidity. To determine the ash percentage, the sample is placed in a platinum crucible, and held over a lamp until all the organic matter is burned out and the ash has assumed a light color. The cold ash is then moistened with a carbonate of ammonia solution, and the crucible again exposed until it is dark red; the weight of the ash is then taken. To determine the percentage of wool, a sample of the paper is dried at 230° Fahrenheit and weighed, boiled in a porcelain dish in potash lye 12° B. strong, and frequently stirred with a glass rod. The wool fiber soon dissolves in the potash lye, while the vegetable fiber remains unaltered. The pulpy mass resulting is placed upon a filter, dried at 212° Fahrenheit, and after the potash lye has dripped off, the residue, consisting of vegetable fiber and earthy ash ingredients, is washed until the water ceases to dissolve anything. The residue dried at 212° Fahrenheit is weighed with a filter, after which that of the latter is deducted. The loss of weight experienced is essentially equal to the loss of the wool fiber. If the filtrate is saturated with hydrochloric acid, the dissolved wool fiber separates again, and after having been collected upon a weighed filter, it may be weighed and the quantity ascertained.