CONSTRUCTION OF RETORTS. 89 



What I shall state here I have from unquestionable authority ; and although 

 I give my opinions with diffidence, I must acknowledge the advantages of the 

 large earthen retorts, as shown in Plate VII., to be considerable. 



One practical point must be observed, that clay retorts of small dimensions 

 are less oeconomical than those of larger size, owing to the great per-centage 

 of fuel required to keep them at a proper temperature for decomposing the 

 coal. The advantage of using the latter description of distilling-vessel is 

 simply a question of profit and loss, or whether it is cheaper to burn iron or 

 coal. The material of which they are formed is a non-conductor of heat, 

 consequently the absorption of caloric is less rapid ; and although they retain 

 their heat when a fresh charge is introduced better than iron retorts, yet not 

 sufficiently to bring down the quantity of fuel as low as that required for metal. 

 Notwithstanding this, even small clay retorts are preferred in many places, 

 particularly in Scotland. Mr. James Reid of the Montrose Gas-works has 

 favoured me with the following description of his earthen retorts : 



" We have had clay retorts in operation for the last three years, and from the great 

 difference in price, compared with that of iron retorts of the same size, and from the im- 

 mense superiority over metal in working them, we have entirely given up the use of the 

 latter. I tried the clay retorts in the shape of an ellipsis, in the D and circular form, and 

 find the cylindrical to be the best adapted for carbonizing the coal effectually*. The size I 

 find best adapted to all purposes is eight feet long, fourteen inches diameter, and four inches 

 thick : such a retort costs at Inverkatling or Clakmannan 2 6s. ; the pillars or columns 

 for supporting them are 6s. each, and each retort finished costs 3 4s. The mouth-pieces 

 are cast metal, and fastened to the end of the retorts by bolts and flanches, as in the ordi- 

 nary description, and jointed with fire-clay and iron cement. The retorts are made in 

 two lengths, and are jointed by a body of fire-clay well diluted with water. The most 

 CEConomical plan for erecting them is to set them three under one arch, heated by one 

 fire. Their only drawback is, that when the heat is let down they contract unevenly on 

 cooling, and are liable to leak when again required for distillation ; they generally last 

 two years." 



Clay retorts have been used for some years by Mr. Eunson at Wolver- 

 hampton with success, the cost of material for setting an oven being under 

 2. The retorts are circular and made in joints of 32 inches long. In several 

 places these retorts are made at the works. 



Of clay retorts, or ovens on a large scale, I have given two examples. 



* Upon this point I should feel inclined to differ with Mr. Reid, for the reasons stated at p. 60. 



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