340 



THE CIVIL ENGINEER AND ARCHITECTS JOURNAL. 



[Nov 



lock of the canal with surh a depth of water that ships iright enter, at 

 least at high water, duiing the neap tides, about the first and last quarters 

 of the moua. 



TANNED CANVAS. 



Communicationii from Mr. Hamlf.t Millf.tt respecting a mode of ren- 

 dering Canvas durable by means of Tun, to the Commissioners on the Fine 

 Arts. 



It may he necessary first to stale that the process of'tanniiie; canvas, 

 althoujih easy and exjiedilioiis, is altogether dillerent from that wliich is 

 emplojed by tanners for converlini; the skins and hiJes of animals into 

 leatlier: it varies also from that mode in comnion use among the \'ai month 

 herring-fishernien and olliers for tanning tlieir nets to pro ect them from 

 prenialure rottenness, allhonnh the process of extrarling the tan from the 

 oak bark is nearly the same as that whi( h is used by the fishermen for 

 the above piM po-e, namely, by decoction. It will he seen from the 

 following stiitement that every possible care and attention has been 

 paid to render the results of the experiments as decisive as could be 

 desired. 



Four iinprimed | canvases were tacked with copper nails to four stretch- 

 ing frames. The four canvases were cut from oil the same roll of cloth, 

 and the stretchers were all manuraclured from the same plank of w .od. 

 Two of the four canvases, after being stretched on their frames, were 

 immersed for three days and nights in a vat of strong tan, made expressly 

 for the occasion ; the tanning li'juor being kept, dnrini; the whole process, 

 at a tPiupcrature of 150°. The two canvases, after being taken from the 

 vat and well dried in the open air, were hung on the side of an under- 

 ground cellar, the walls of which were never free from damp ; the floor 

 was often tii>odeil in wet seasons for months together, and the water, long 

 before it disappeared, never faded to become putrid. This circumstance 

 rendered the situation in which the tanned canvases were hung for trial 

 peculiarly well adapted for ascertaining the power of the antiseptic 

 properties of tan. The two uuianned canvases, after being tacked with 

 copper nails to their stretchers, were hung up near the two tanned ones. 

 Care being taken to exclude all circidation of air, the cellar door was 

 locked, and the four cau\ases were entombed without once seeing the 

 light for no less a period than ten years. 



Mr. iMdlett proceeds to state that when, after the trial, the four canvases 

 were removed from the cellar into the light for examination, the two tanned 

 canvasses were found lo have completely resisted putrefaction, no symp- 

 tom of decomposition appearing in any part of them. Fliers were forcibly 

 applied to the edges, but the cloths were found to be as strong as they 

 were on the day when ihey were tirst placed in the cellar. The stretchers 

 also, which were tanned with the canvases, were found lo be no less 

 sound ; they were piei'ced with a gimlet in several pans, both back and 

 front, and wire found to have undergone no change. Not so with the 

 untanned canvases and their stretchers ; in them decomposition had maile 

 very considi rable progress; they were found to be in a half stale of decay. 

 On using the pliers the canvas proved to be extremely tender, rending 

 with little exertion. The stretchers also were in a state of decay; on 

 piercing them with a gimlet they were observed to be very unsound; and 

 the growth ot fungi, one of the evidences of decomposition, showed itself, 

 particularly about the joints of the stretchers. The same two tanned 

 canvases were again placed in the cellar, together with one of the two 

 untanned canvases ; the other, after having been cleansed (as well as its 

 stretcher) froiu the mould that covered it, I caused to be tanned precisely 

 in the same way in which Iha other canvases had been treated. After 

 being taken from the vat and dried in the open air, this canvas was also 

 placed in the cellar with the other three, and the door was again closed on 

 them. 



These experiments, Mr. IVlillett adds, were continued with the same 

 canvases during a considerable lapse of time, in all ahcmt twenty years. 

 When finally examined, the two taioied canvassses, together, with their 

 stretchers, continued, notwithstanding the severe trial they had exprri- 

 enced, sound in every part; not the smallest symptom of decay could he 

 discovered ; proving, far beyond my anticipations, that the antiseptic pro- 

 perties of tan are sufliciently potent to resist putrefaction and decomposi- 

 tion in canvas, and even in wood ; hence, rendering them durable even 

 when placed in the most trying situations. 



The untanned canvas and its stretching-frame were found, on their final 

 inspection, lo be completely decomposed; the canvas dropped from the 

 frame, and the frame itself was a mass of touchwood. The half decom- 

 posed canvas and stretcher which I had caused lo be tanned before the 

 second trial look place, as above stated, had suH'ered no further decompo- 

 sition ; a strong proof that tan has not only the power lo prevent but lo 

 arrest the progress of decomposition. This latter experiment was ex- 

 pressly on a supposition that, if the result should prove successful, Ihe 

 process might be rendered available in prolonging the existence of the 

 valuable works of the old masters. 



Process for extriiclin^- tan from oak bark for tanning canvas. — One hun- 

 dred weight of good, sound, and conrsely-ground oak bark, will make one 

 hundred gallons of proper tanning liquor, which is sufficient to tan about 

 sixty scpiare yards of canvas. 



To make a vat of tan of any required si:f.— Take any quantity of good 



coarsely-ground oak hark, according to the number of square yards of 

 canvas about to be tanned, and water agreeably to the foregoing propor- 

 tions ; boil them together for twelve successive hours in a copper vessel 

 (not in an iron one, as Ihe oxide of iron has a tendency to decompose tan), 

 taking care to supply the decoction with fresh water from time to time to 

 make up the loss occasioned by evaporation. When sufliciently boiled, 

 strain oil' the tanning lii|uor from the bark through a hair-sieve, and put it 

 by for use. The tanning liquor thus made is at least three times as strong 

 as that made made use of by the herring fishermen, to render their nets 

 durable, and still stronger than that employed by the Dutch shipowner* 

 for tanning the sails of their vessels with Ihe same view. 



The copper in which the oak bark has been boiled, will serve every 

 required purpose as a vat, with one trilling addition, namely, a small piece 

 of Hat oak board, about nine inches in iliameter, and one inch thick, cut 

 into a circular shape, lo be pliice'l at the bottom of the inside of the cop- 

 per. Its use is to prevent the canvas, whilst undergoing the process of 

 tanning, from touching the bottom of the vessel. 



Process fur tanning the canras. — Take any quantity of canvas, say 15 

 yards, to tan which 25 gallons of tanning liquor, made as above are re- 

 quired. Hour the tanning liquor into >our vat or copper, and heat it by 

 means of a gentle lire uniler it, to the temperature of 150°, then immerse 

 the canvas therein, and continue the immersion at the same degree of tem- 

 perature, day and night, for 48 hours. A longer period would do the 

 canvas no harm; a shorter is by no means recommended. The canvas 

 during the process of the tanning should now and then be moved in the 

 vat, in order that every part of it may be equally tanned. The tanned 

 canvas, on being laken from the vat, must be neither wrung nor squeezed, 

 but hung up or laid out on a grass-plot to dry, allowing the air alone to 

 take up the remaining moisture. 



The process for tanning canvas, when strained on stretching frames, is 

 dissimilar to the foregoing, both as to the construction of the vat, and the 

 mode for regulating and keeping up the necessary temperature required 

 for carrying on the process of tanning the canvas already strained. The 

 process about to be ilescribed rajght indeed be dispensed with, as canvas 

 can be as readily strained on the stretching-frames after as before it bai 

 undergone the process of tanning, and much labour and expense would be 

 saved thereby ; but it has been thought that the stielching-fiaines on which 

 the canvas is strained, ought to receive from the tan the same protecting 

 advantages as the canvas itself, they being equally subject to the causes of 

 decomposition. Stretching frames intended lo undergo the tanning pro- 

 cess, ought to be completely made and fitted, but not put together before 

 they underao the process ot tanning. It is scarcely necessary to observe 

 that the stretching-frames here mentioned are those to which canvas is not 

 yet attached. 



To construct a rat for fanning canvas when stretched on its frame, togethir 

 with the aiijmrnlus for heating the tanning liquor, and keepnig it at a proper 

 temperature during the operation of tanning. — A case made of oak plank- 

 ing, about an inch in thickness, and somewhat resembling a picture pack- 

 ing-case, was provided, with one side left open to admit the strained can- 

 vases ; this served as a vat. The case or vat being placed edgewise, a 

 lead worm of a20-gallon still was wound round Ihe inside of it, and steam 

 from water, constantly kept in a boiling state in the still, and forced through 

 the worm in the vat, kept the tanning liquor at a proper temperature. This 

 was the sort of vat which was used for tanning the two ^ canvases and 

 their stretching frames, before mentioned; it was expressly made for carry- 

 ing into efiect those experiments. 



Jlr. Millett proceeds to offer some observations on the mode of lining 

 pictures: — A picture requiring to be new lined, being spread oul for that 

 purpose, take a basin of strong tanning liquor, prepared according to the 

 foregoing process, and in a moderately warm stale ; wash the back of the 

 picture therewith, using a soft sponge ; when the tanning liquor so applied 

 shall have become dry, the same process should be repeated, and so con- 

 tinued for three or four times. The use of the tanning liquor so employed, 

 is to arrest, by the antiseptic properties of tan, Ihe further progress of the 

 decomposition which may have taken place in the original cun\as on which 

 the picture is painted. The second precaution to be observed is lo apply a 

 well-tanned canvas as a lining to the picture ; and lastly, as a still further 

 protection to it, it would be well to use a stretching-frame that has under- 

 gone the process of tanning also. 



A vat or apparatus constructed as follows, will be found well adapted for 

 the purpose of tanning stretching-frames: — A square trough of any re- 

 quired length, and about a foot in height and breadth, formed of zinc and 

 lined inside with oak planking, with a lid made to open and shut, is all 

 that is necessary as regards the vat itseli. To heat the tanning liquor 

 therein and keep it at a proper temperature, during the operation of tan- 

 ning, a lead-pipe leading from a steam-boiler, and passing through Ibe 

 tanning liquor in the vat, is also required. The heating process should be 

 regularly kept up for at least 48 hours. The stretching frames should be 

 regularly maile and tilted previously to their being tanned, and they should 

 be tanned before they are finally put together. 



The writer further remarks that few things are so destructive to canvas 

 as oil, and expresses his opinion that, in priming canvas for painting, care 

 should be taken to have it well sized, so that it may fully resist the oil in 

 the priming colour. 



