London
122 Regent St
Apl 29th 1850
To H F Talbot Esqr
Sir
Sir David Brewster <1> is I am told staying at Woolwich I do not know where, or for how long. I will try to find out.
I think it desirable to use a Barytes salt after the Potash or with it. Barytes itself would probably destroy the Hyposulphite it will certainly take down most of the sulphur salts for this reason I advise it as the Potash acts by converting one sulphur salt into two or three & those harmless I am inclined to use an Acid Nitric or Sulphuric very weak after the Potash &c to liberate destroy any sulphuret that may be formed These are only theoretical remedies to meet theoretical objections The analyses required to determine absolutely the state of the case are so delicate & intricate that they must be left for leisure moments.
It is perhaps enough to say that Dr Lyon Playfair <2> thinks the Potash a necessary precaution The change of colour alone produced by it is worth having & being itself harmless no objection can reasonably be raised If after all we have not fixed the pictures by one application of the Potash we have done much towards it. The discovery that Hydrosulphate of Ammonia fades them is important. It first blackens them. If our brown pictures become black – immerse them in the Potash to take away the sulphur They will then last until black again then again Potash them. Daguerreotypes want cleaning with cyanide of Potassium occasionally. <3>
I find varnish can be applied so as not to displease artists. this should be used.
Yours obediently
T A Malone
I intend casing the Portraits with a cement containing Carbonate of lead this absorbs Hydrosulphate of Ammonia & will preserve the colours of the Artist.
I do not think the Sulphur will penetrate far into the lead cement for Portraits Mortar is porous yet caustic lime has been found in the centre of a mass of it after 500 years The Carbonic Acid had not penetrated.
Notes:
1. Sir David Brewster (1781–1868), Scottish scientist & journalist.
2. Sir Hugh Lyon Playfair (1786–1861), military & provost of St Andrew’s University.
3. This was a method long used for removing the tarnish that obscured daguerreotype images over time. It was later replaced with a thiourea compound, which was safer for the restorer but also damaging to the image, and both methods are now frowned upon.