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The action of the vapor of the ammonium hydrosulphide will cause the obliterated writing to slowly turn brown, then black. But within a short time after removal from the box the writing will again disappear. Another method is to wash the document carefully in a solution of hydrochloric acid, one part, and distilled water, one hundred parts.
A net of fine white silk or cotton threads is stretched across the box at about one half the depth. Two saucers containing yellow ammonium hydrosulphide are placed in the bottom of the box. By means of a clean sponge or brush, moisten the paper with distilled water; then place it on the net with the writing side down.
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