Faraday to Charles Lock Eastlake   23 January 1858

Royal Institution | 23 January 1858

My dear Sir Charles

In addition to your letter of the 13th instant¹ you sent me three specimens of cloth, steeped respectively in solutions of Microcosmic salt,- silicate of soda, and chloride of zinc.- the first burns freely;- the second nearly as well and not merely with flame, but also as tinder:- the third may be considered as incombustible. All of these contain salts in the soluble state; they are removable by a slight washing, and then the specimens are left as combustible as if they had never been prepared. The insoluble silicate remaining in the second, did not interfere with its burning.

Yesterday we looked at certain pictures which have been backed with silicate and chloride of zinc cloths². The application of the tongue, shews at once the presence, and the solubility, of the respective salts. In my letter of February 1857³ I objected to soluble salts in the back lining; and I do so still:- for though I cannot say that I am sure evil would result, I think it possible that it might. A soluble salt has an amount of diffusibility such, that, under the influence of varying circumstances and of time; portions may find their way into the frame, and even into the picture. I therefore prefer fixed matters that cannot thus travel. Nevertheless the pictures that have been lined might be left safely for six or twelve months, being examined from time to time, to see if there is any travelling of the salts in the lining, or any other recognizable effect produced, good or bad.

You yesterday shewed me a piece of animal parchment paper; which I think a very excellent material, though it is more combustible than the chloride of zinc cloth. Woollen fabrics, of which there are some, light and cheap enough (see the accompanying samples); are more combustible than the parchment paper, but will not catch fire from sparks. With a coat of earthy matter, analogous to white wash, I believe they would resist both sparks and small flames.

I am | My dear Sir Charles | Your Ever faithful Servant | M. Faraday

Sir Charles Eastlake Bart | PRA | &c &c &c