4 o6 COAL TAR 



The results show that higher temperatures seriously diminished the amount of 

 the more volatile distillates upon which the value of the tar largely depends. 



In the first few years of this century the reduction in England of the Parliamentary 

 standard of illuminating value, and the introduction of the No. 2. standard Argand as a test 

 burner, led to a further pressing of temperatures in order to obtain a large volume of gas, 

 and brought about such a deterioration in the quality of the tar that the market price fell 

 to |d. a gallon; and had it not been for the enormous demand for tar that arose at this period 

 for preparing the surface of roads tar would have fallen probably still lower, as it contained 

 the minimum of valuable products and anything up to 30 per cent of free carbon, which 

 deteriorated the pitch. 



Luckily for those industries that are dependent upon tar products, a marked improvement 

 in the quality of the tar. has taken place with the introduction of vertical retorts, fully- 

 charged horizontals and chamber carbonisation (see under GAS MANUFACTURE), as is> 

 shown .by the following analyses: 



Dessau Continuous 



verticals. Verticals. 



Specific gravity ; :. . . . , r; v . ; >. 1.113 I 74 



Free carbon . . . . 4.0 3.1 



Distillate up to 170 C. . . .... . . 13.0 9.1 



" 270 C. ... .''.... 24.0 25.7 



" "350 C. . :.v ,;. I . -.- . . . 9.0 24.9 



Pitch . . . . : . ; . . : "; . : .' . . 50.0 46.0 



Road Tarring. The use of tar for road surfaces now occupies so much attention that a 

 short description of the methods employed will be of interest. 



The tar used should be coal-tar, either from gas-works or coke ovens, and should be 

 partially distilled before use, in order to get rid of water and volatile oils; the specific gravity 

 should be 1.2 to 1.25 and high temperature tar, containing much napthalene and free carbon, 

 should be avoided, as these bodies have no binding effect and lead to disintegration. 



Crude tar should never be used for surface work; the complaints which arise from the 

 contamination of water in streams, &c. from tar-treated roads nearly all arise from this 

 source, as the trace of water and ammoniacal liquor together with the volatile oils prevent 

 rapid drying, and if rain falls shortly after the tar has been used, slight emulsifying of the 

 surface takes place, and the water that flows into the gutters and streams becomes contami- 

 nated and shows itself by films on the water. The presence of water in the crude tar is also 

 a serious drawback, because the porous road material absorbs water more quickly than tar, 

 and this weakens the adhesion of the tar and leads to crumbling of the surface. Where 

 partially distilled tar cannot be obtained, the best way to prepare the crude tar is to heat it 

 in coppers to a temperature of 65 C. (150 F.) for several hours, when most of the water will 

 be driven off, but great care must be taken in doing this, as if the temperature rises above 

 this point frothing over of the tar and ignition from the fire below is very apt to take place. 



There are three methods by which the tar may be used in preparing the surface of the 

 road: (i) Simply tarring the surface, as is done in most rural districts; (2) Making the sur- 

 face of the road with tar macadam; (3) Grouting the macadam whilst making the road with 

 tar. The first method is the one employed to the largest extent, and depends for its success 

 on choosing a period of dry weather to carry it out; the surface of the road must be dry and 

 all dust removed, the tar is then run on, preferably hot, from a tank on wheels with any form 

 of sprayer that will give an even distribution, or it may be applied by hand, which allows 

 of more attention being paid to local conditions in the road surface, and sand or stone screen- 

 ings may be dusted sparingly over the surface of the tar, and all excess swept off when the 

 tar sets. The advantages of this system are low cost, suppression of dust, increased life of 

 roadway, economy in watering and good surface, whilst trie disadvantages are that it gives 

 a slippery road for horses with frost, and if the tar contains too much free carbon and naptha- 

 lene, a black mud in wet weather. The second system consists in mixing the macadam with 

 the prepared tar before making the road. Limestones absorb a very much larger proportion 

 of tar than granites and make a more adherent mass when rolled, but granite road metal 

 can be used quite successfully. The procedure is to grade the road metal roughly into 2\, \\ 

 and \ inch sizes: these are well dried and are mixed with the prepared tar and made up into 

 heaps that are allowed to stand for a week or ten days, and a layer of the 2\ inch metal is 

 put down and rolled, the operation is repeated with the \\ inch metal, and the surface is 

 made of the \ inch size. The third or "grouting" system is to put down the macadam in the 

 usual way, but dry, and after light rolling, to mix hot prepared tar with sand and float it over 

 the surface, brushing the grout into the interstices of the macadam, and lightly sifting a little 

 screening over the surface. The great demand for coal-tar for this purpose has created a 

 shortage in the supply, but the improvement in price will probably lead to a rapid extension 

 in the use of coke oven recovery plant, thousands of tons 01 tar being burnt to waste annually 

 in the beehive coke ovens, which still are used in the North of England to a very great 

 extent. (VIVIAN B. LEWES.) 



