If the paper were left as it exists when the scratching or rubbing is completed, it would be very easy to see that it had been tampered with, for not only would the parts thus abrased show the running of any fluid which was subsequently laid upon them, but the surface would appear rough to the eye in comparison with adjacent parts of the paper, and the place would appear thinner by transmitted light.  Even to the touch the surface would reveal differences from the ordinary condition of other parts of the paper.

But the forger usually endeavors to overcome these difficulties by applying to the scratched area sandarach, resin, alum, paste, or two or three of these together, the effect being to prevent an unusually large flow of ink from the pen and its abnormal absorption by the paper.

The paper should be placed between the observer and a strong light, by which means, either with or without a magnifying-glass, a distinct increase in the brightness of the suspected area may be noticed, indicating a thinning, and even traces of letters, or marks which have escaped the erasing-tool, may be seen.

A close scrutiny may show places where the surface has been partially torn, and the fibres of the paper united together into little knobs, and almost invariably a magnifying-glass will clearly show the disturbance of the superficial fibres, as compared with other and normal parts of the paper.  If the latter be tinted, the change of appearance may extend to color.  The color of the paper should always be attentively observed.

A change of color over the part which is the subject of investigation may indicate the mechanical removal of the paper itself, or a washing either with water or with acids, alkalies, or saline solutions.  A certain spotted character which follows this latter treatment differs from the changes of color due to age or soiling.

When the heavier strokes—­usually the down strokes—­of a writing are thicker and more blurred than usual a removal of sizing is indicated, or an original imperfect sizing of the paper.

On the contrary, where the strokes are thinner and closer together than usual, the cause is generally the application of resin, which has been added, in all probability, to conceal a previous scratching of the surface.

The spots produced by washing are more like penumbra, or blurred marks bordering the tracings of the character, and are generally colored.

In order to bring out any traces of ink-marks which have been so far removed as not to be observable by the naked eye, Coulier recommended the placing of the document between sheets of white filter paper and passing a hot flatiron over it, allowing the latter to remain on the spotted parts for a short time.  Another method is to wet the suspected paper or document with alcohol, wrapped in another piece of paper also saturated with alcohol, for the purpose of bringing out as yellow rusty marks all the pen strokes which had not been entirely removed by erasure.