alnovo@inwind.it
Date: 07/30/01-04:02:37 AM Z
> ... as potassium bisulfate can be used
> instead of potassium bisulfite or metabisulfite (or vice versa) and/or
> sodium sulfite (tho I doubt sodium sulfate) for clearing gum, but I doubt
> most. Of course if there's someone on line who knows for sure, I take that
> back. (Which ones were you thinking of?)
>
> Judy
>
>
bisulphate and bisulphite ARE NOT the same thig, and they have quite different properties.
Pot. bisulphate, KHSO4, can be thought as a 1:1 mixture of pot. sulphate and sulphuric acid. Its reaction is strong acidic.
Pot. bisulphite, KHSO3 (and metabisulphite, K2S2O5) can be thought as a mixture of pot. sulphide and sulphurous acid. Its reaction is weakly acidic and, owing to the presence of sulphite ion, reducent (that is the contrary of oxidant).
However, as for clearing gum someone suggests sodium bisulphite and other suggest sulphuric acid, the question is: what is better to clear gum? A weak but reducent solution or simpluy an acid one?
The first works because the yellow staining of dichromate is reduced to trivalent chrominum, and perhaps it is carried in solution by the weak acidity. Usually are recommended 5% solutions.
The second works because the trivalent chromium is *surely* dissolved by the strong acidity, and perhaps the residual dichromate undegoes to a reduction to trivalent chromium, reacting with some reducent yet present in the paper, like the gelatine and the gum themselves. Note that the reduction of dichromate needs strong acidity.
If I remember well, there are suggested 1% solutions of sulphuric acid, but it is known that acidity is not the best for an archival purpose.
In my opinion there is also a third way: a solution of sodium sulphite (say 0.5%) with some sulphuric acid added (say 0.1%). The clearing time is about ten minutes, then wash 5' in tap water, then 5' in 0.5% sol. of sodium carbonate (to be sure of neutralizng the sulphuric acid), then two rinses in tap water.
Alberto
(I will have my holidays in a few days, far from my PC and internet. For this reason I am going to temporarily unsuscribe)
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