I wonder about the volume of sensitizer and the surface area sensitized.
When I was doing tricolor carbon I found I had to use a one-shot sensitizer
for constant results as the third sheet had significantly more contrast and
less speed than the first, especially with a weak concentration of
dichromate sensitizer as the dichromate ion was absorbed gradually as the
sheets went through it. This was a serious problem with low concentrations
of dichromate, e.g., 1% although I never went below that.
>
> b) The same batch of carbon tissue was used for
> all tests. This was a warm brown tissue made=20
> from 3 parts Carmine Crimson, 4 parts Brown, 3
> parts Primary Yellow, and 2 parts Sumi Ink=20
> concentrate.
This is a fairly low contrast color to start with.
>
> c) Exposure was to a #2 Kodak step tablet.
> For exposure the tissue was placed 4 inches=20
> from the light source, in a bank of tubes=20
> separated from each other by about 1/4 inch.
> Tests were made with all UV tubes (UV), a
> combination of UV and Cool White tubes=20
> (Comb) or all Cool White tubes (White).
How "fresh" were the tubes? New, old?
>
> d) Exposure was 20 minutes, in all cases enough =09
> to produce two steps of maximum density.
..
>Test One (Comparison of Sylvania UV tubes and GE Cool White tubes at sens=
>itizer strength of 1% and 1/2%. *My experience with 350na tubes in carbon=
> printing is that they are very sensitive but produce images very low in =
>contrast unless used with very dilute dichromate solutions, which of cour=
>se in turn lengthen exposures somewhat.
>
>sden 1/4% 1% =09
> UV Comb. White UV Comb. White
>1. 1.73 1.44 1.38 1.30 1.48 1.40
>2. 1.67 1.40 1.32 1.26 1.44 1.34
>3. 1.52 1.31 1.21 1.22 1.38 1.27
>4. 1.49 1.29 1.07 1.17 1.28 1.24
>5. 1.40 1.23 .96 1.14 1.18 1.13
>6. 1.32 1.06 .81 1.07 1.14 1.05
>7. 1.21 .96 .59 1.06 1.10 .88
>8. 1.13 .84 .37 1.00 1.05 .74
>9. .99 .63 .13 .90 .93 .49
>10. .81 .49 .11 .81 .85 .19
>11. .65 .31 .08 .71 .76 .14
>12. .49 .13 -- .66 .67 --
>13. .29 .11 -- .62 .53 --
>14. .15 -- -- .57 .44 --
>15. .05 -- -- .38 .35 --
>16. -- -- -- .36 .27 --
>17. -- -- -- .16 .12 --
>18. -- -- -- .11 .10 --
>19. -- -- -- .08 .08 --
How much time (hours?) did it take you to go from the first test to the
last one?
>
>Test Two (Expose with Cool White Tubes alone, sensitized with 2%, 3% and =
>5% potassium dichromate. Exposure for 20 minutes). Compare UV exposure wi=
>th 1/4% sensitizer with Cool White exposure at 5%!!!
>
>sden 2% PD 3% PD 5% PD
>1. 1.58 1.60 1.76
>2. 1.55 1.56 1.73
>3. 1.46 1.44 1.65
>4. 1.29 1.39 1.46
>5. 1.22 1.24 1.36
>6. 1.09 1.17 1.28
>7. .93 1.02 1.16
>8. .78 .95 1.04
>9. .62 .86 .93
>10. .42 .68 .81
>11. .34 .62 .68
>12. .29 .54 .61
>13. .20 .26 .56
>14. .13 .20 .34
>15. .10 .12 .16
>16. .08 .08 .13
>17. -- -- .08
>18. -- -- --
> =09
> =09
>
>
>
>My conclusion:
>
>1) When the dichromate sensitizing solution is adjusted to the exposing l=
>ight the Cool White tubes are as effective in exposing carbon tissue as 3=
>50na UV tubes.=20
>The characteristic curve plotted from tissue sensitized with a 1/4% potas=
>sium dichromate solution and exposed with 350na UV tubes is virtually ide=
>ntical to that plotted from tissue sensitized with a 5% potassium dichrom=
>ate solution and exposed with Cool White tubes. The results from my plott=
>ing are:
>
> UV (1/4%) Cool White (5%)
>Dmax 1.56 1.59
>Dmin .09 .11
>DR 1.47 1.48
>Av.Grad .88 .77
>ES 1.68 1.91
>Sp Pt 0.65 0.67
I would never have thought that the contrast difference between the light
sources would have been that great. Very interesting.
..
>2) When the tissue is sensitized with dichromate solutions of 1% or more =
>there is a strong suggestion that a combination of UV and Cool White tube=
>s is more effective than either along. I have conducted some further test=
^^^^^^^^^^^
effective in what way?
>s to confirm this but don=D5t want to bore you with any more details for =
>now. I base this observation in part on my belief that more even results =
>are obtained in carbon printing when the strength of the dichromate sensi=
>tizer is 1% or stronger.=20
Most definitely, which is why I seldom used less than 2%. The sensitizing
time is also very important; perhaps more when dealing with weak
concentrations. This said, I don't see how the latter part of your
statement affects the previous part.
Luis Nadeau
awef6t@mi.net
Fredericton, New Brunswick, Canada
http://www.micronet.fr/~deriencg/nadeau.html