J. F. Campbell Collection > Method of converting collodion negatives into positives, by heat

METHOD. &c.
In the end of 1854, I observed that a broken
glass negative changed colour on being thrown
into the fire.
I then tried several experiments with small
negatives. I held the glass by one corner with
the tongs, and passed it gradually into an
ordinary coal fire, selecting a place between the
bars of the grate where there was a good red
heat and no liame or smoke. By carefully and
slowly drawing the glass out after it had got to
a red heat, by avoiding contact with the fuel, and
by holding the collodion side of the plate down-
wards to avoid dust, I succeeded in producing a
number ofpositivepictures,which I varnished and
backed with black varnishin the ordinary manner.
Many glasses were broken in ray first attempts.
Some pictures were too pale, some were yellow,
some were too white ; but the defects in those
which escaped breakage appeared to be due
rather to the chemical tlian the burning process,
and 1 persevered.
I have made a number of experiments since
1854, and the result is the following process,
which I have found tolerably successful.
Process.
Take a picture by the ordinary collodion process
on plate glass, which stands heat better than the
other kinds usually sold for photographic pur-
poses. Carefully remove all traces of iodide of
silver, which gives a yellow colour in the shadows
if any is left; dry,and varnish with amber varnish.
The negative may now be used for printing
in the pressure frame. To convert it into a