From: pete (temperaprint@blueyonder.co.uk)
Date: 02/08/03-02:03:52 PM Z
Well done Dan I havenšt done the math but always suspected the double sheet
glass method.
Dave,
Great idea using a vertical positiong of the lights a usefull saving in
space
Here is another method for consideration
This an extract from an article I wrote for the APR some while back, please
forgive the commercial slant but it did address the underlying problems of
getting fine definition in contact printing at the time of writing.
ITS A FRAME UP Peter Fredrick
The LITESTAR pneumatic system a new
approach to precision contact printing
Contact printmaking is an essential component of Photo-Alt activity, most
of the systems use techniques, that stem from methods
and equipment worked during the last century. These frames are based on
design that dates back to the POP days, when the images tended to be small
by modern standards. A whole plate [8.5"*6.5"] image was considered large,
cabinet was the norm [6"*4"] and the postcard was very popular. Negative
substrates were made of glass,small in size and sharpness could be easily
maintained.
However the use of line , digital, and stochastic screen methods working
with film require contact systems that can deliver pin sharp definition ,
coupled also to the problems associated with the increasing size of modern
image making, a fresh approach is needed.
CONTACT PRINT MAKING
In contact printing, the basic underlying principals are in essence simple.
All that is needed is a piece of glass and a rigid support. The glass
is placed on top of a sandwich that is made up of the glass, negative, light
sensitive print material and rigid support, this forms a sandwich that
must be held tightly with suitable clips/clamps so that the negative and
print material are in perfect contact.
DESIGN PROBLEMS
The main design problem when constructing a contact printing frame is to
ensure adequate contact of the negative and material to be printed on. If
adequate contact is not maintained, fuzzy, out of focus areas and spots will
result. An overall loss of sharpness will also occur, which can sometimes go
undetected by the Photo-Alt printer, who may blame the negative instead of a
faulty contact frame.
The secret of successful contact printing depends on pressure, which has to
be even and strong. Most frames with a spring back do not give sufficient
evenness or strength of pressure. Only when the spring is large can
sufficient pressure be exerted. Even so most of these frames leave much to
be desired in terms of contact quality.
As pressure increases, other problems arise. The frame must be made very
sturdily otherwise the pressure can, over a period of time, split the
corners of the frame. In addition, too much pressure can break the glass.
COMMERCIAL FRAMES
The large size contact printing frame is a fairly specialist item and may be
difficult to obtain. Kodak market an excellent frame, 28 x 36cm (11 x
14in.), which has the added advantage of an integral punch register system
if used with the Kodak register punch. This is, however, very expensive,
Paterson also manufacture photographic contact frames that are primarily
made for negative proofing in 35mm 2 1/4 sq. sizes, but they do a slightly
larger frame that takes four 5 x 4 inch negatives and can be used as a 10 x
8 in. contact printer.
Most of the large frames on the market relate to commercial practice with
the photo-mechanical printing industry, in mind, and are therefore
relatively expensive, also not particularly friendly for the small scale
user.
Having said this, however for the larger sizes it is far better to use a
vacuum frame, which is a highly efficient method of contact printing. but
as said before the apparatus can be expensive, heavy, noisy, and cumbersome.
SIZE PROBLEMS
A normal springback frame works very well up to approx.10ins*8ins in size,
but after this there is often some loss of fine definition.
The development of the pneumatic frame arose out of a system of print making
developed at Kingsway College Kentish Town centre.
A slightly soft photgraphic contone print is made which is then reproduced
using a quality colour copier as a monochrome print, the print should look
identical to the original, but in fact on close scrutinty is seen as an
image consisting of a series of fine black lines, this repro-print is then
contact printed onto lith/ line type film.
This method is very effective, and produces a negative that prints easily on
most of the alternative photographic print processes. but does need
absolutely precise contact, in its manufacture, As our preferred image size
has tended to move up to A3 we started to run into contact problems.
THE KINGSWAY SOLUTION
Adversity is said to be the mother of invention, so back to the drawing
board, and many complex methods were tried out, foam, toggle clamps, which
half worked, and Gee clamps which did, but resulted in a lot of broken
glass.
Then one day I had a brain storm it must have been that the moon was in the
right quarter or a bad pint of beer !.
Why not a reversed vacuum, i.e. blow up instead of suck out, a much simpler
mechanical concept to achieve from a practical viewpoint, than trying to
maintain a vacuum. A small hand pump would suffice such as used for blood
pressure testing, and silent operation is guaranteed, after a few
development problems the system was found to work excellently. and was
tested out on my students, when it worked so well, that a number were
purchased and THE LITESTAR PNEUMATIC PRINTER were born and eventually
marketed by Martin Reed of Silverprint ltd the English version of Bostick
and Sulivan. I stopped making them some years ago due to increasing senility
! despite this the frames are still working well even if I am not.
I hope this helps
Pete
>
>
> Judy said in her message...
>
>> Then, I generally put a gallon jug or two of water on the top -- which
>> takes all of two seconds... contact is excellent. And quiet.
>
> First off, I don't question for a moment that Judy and others have had
> great results getting good negative/paper contact with the glass-sandwich
> type frame. If your printing paper is fairly immune from curl after
> coating, you might get by just fine with a glass-sandwich frame. My
> personal experience is different.
>
> Ten years ago when I started making 12x18 platinums, I constructed one of
> those glass honkers, feeling confident that such a heavy sheet of glass
> could tame any marriage of paper and negative. Boy was I wrong!
>
> Do the math on the water jug example: water is approx 8 lb/gal so with
> two jugs you have 16 lbs. If your glass is really beefy, it might weigh
> 10 lbs. Now you have a total of 26 lbs. or 416 ounces of glass and jugs.
>
> Now say you're using 16x20 glass for your frame, which makes a total of
> 320 square inches (though if you've designed your glass frame with enough
> space for two gallon jugs, you probably have much more than that in
> surface area).
>
> Assuming the conservative 16x20 size, you do the math (dividing 416 oz.
> by 320 in-sq and you find that you are really only getting 1.3 ounces per
> square inch. That's nothing when it comes to flattening wavy paper
> products. Of course with larger frames your weight per inch goes down
> even further.
>
> A vacuum frame creates 14.7 POUNDS of force per square inch. There aren't
> many materials that can argue with that (in fact, you'd have to pile
> nearly 600 gallon jugs on the glass to equal that figure).
>
> Vacuum frames can be had very cheaply on the used market. The pumps are
> noisy but the noise can be reduced by placing the pump in an adjacent
> room (the frame, after all, is silent) or by insulating the pump from the
> floor with vibration reducing material, like carpet padding. Much of the
> noise is caused by the pump's vibration feeding into floors or cabinets,
> acting much like a not-so-sub woofer.
>
> You could try the two sheets of glass method first to see if it works for
> your process. If it doesn't (and you don't want to buy new or build
> yourself) check the "printing supplies" section of the yellow pages and
> start asking about used vacuum frames.
>
> Just don't ask me why on this beautiful praying-for-snow-in-Texas morning
> I'm doing math about gallon jugs and vacuum!
>
> Hope this helps,
>
> Dan
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