RE: Speed Point in PT/PD printing?

Eric,

Thanks for the reference. I will get a copy of the article and try to
determine if any of the data is relevant to my tests.

I do have on hand ferric oxalate powder from two different sources.
Ferric oxalate has been described as a "somewhat undefined substance"
so the control check sounds like a good idea.

Regarding UV blockers, do you mean things such as film base and glass?

A couple of years ago, right after I finished the article on UV light
sources that is now at www.unblinkingeye.com I acquired a few sample
pieces of Starfire glass and made some tests with it in the small
contact printing frames that I use to expose Stouffer 4X5
transmission wedges. Starfire indicates that their glass transmits
between about 3-6% more light than regular glass in the range of
400nm to 500nm.

In my tests I was not able to detect any difference in printing speed
with either dichromate or iron processes. However, the glass used in
my tests was only 1/16" wide and if one were to make the same test in
a large printing frame with 1/4" or 3/8" thick glass the results
might very well be different because there is no doubt but that a
fairly significant percentage of UV light is blocked by regular plate
glass. I measured the transmission loss in the glass in one of my UV
printers by taking a reading in UV mode through the densitometer,
with sensitivity at 373nm. The density readings transmitted to a loss
of log 0.16 for a 1/4" thick piece of glass. That figure represents
a speed loss of a full 1/2 stop in printing speed, or a loss in
transmittance of around 30%. However, even assuming that the Starfire
glass increased transmittance at this wavelength by 5% over regular
glass of the same thickness this would translate into a speed gain of
only about log 0.5, or 1/3 of a stop.

Sandy

Sandy

>Sandy, The data that I was referring to is on Page 15 of Vol. 34, 1986 of
>the Journal of Photographic Sciences. I was mistaken, he includes
>wavelengths down to 254nm and up to 620nm in a table giving Quantum Yields
>for the Photolysis of Aqueous [Fe(c2o4)3]3-.
>
>I would recommend that you try several "brands" of Ferric powder to check
>the 25% solution. If you mix B&S powder, or any powder for that matter, the
>best you can hope for is a 25% solution from that powder as the ferric does
>vary from maker to maker and batch to batch. I have seen speed variations
>of +/- 10% when switching batches of ferric powder.
>
>Are you mentioning glass or other UV blockers within the test?
>
>Eric Neilsen Photography
>4101 Commerce Street
>Suite 9
>Dallas, TX 75226
>http://e.neilsen.home.att.net
>http://ericneilsenphotography.com
>
>
>> >> >> >>Any thoughts on this by pt/pd printers would be appreciated.
>> >> >> >>
>> >> >> >>Sandy King
>>
Received on Wed Nov 12 16:08:09 2003