View Full Version : 24 X 38 inch contact printer ideas, discussion, users?

Randy Moe
4-Sep-2016, 18:32
Yes, I know I come up with hair brained ideas, I always have, no reason to stop now. I used to get paid well for it.

I have been working with 14X36 inch X-Ray film as enlarged neg.

I was just given 6 healthy 48 inch BLB bulbs in good fixtures. I tried to sell them for the kind lady, but no bites, so...a free item turns into another project.

I looked up 1/2"-16x38" tempered table top glass. Cheap delivered!

I have a pallet rack shelf I could turn into a big contact printer. I think the glass is heavy enough to flatten most things on a foam base. I may need spring lift assist. That's easy, and the rack can hang the bulbs above on cables that lower them after glass is closed.

Not planning on vacuum for now, but that is also doable if necessary. TBD

Then I looked up rolls of Ilford SG paper, so instead of 16 X38" inch glass for a 14X36" negative, I may as well make this contraption big enough for roll paper. https://www.bhphotovideo.com/c/product/447070-REG/Ilford_1769405_Multigrade_IV_Deluxe_Black.html

Has anybody seen a rig like this or used one?

Does tempered glass pass UV well?

Is 1/2" glass going to cause distortion or softness.

If Ilford quits rolls, is there good Alt print paper this big?

As Bob Carnie is gearing up for digital to film on rolls, negs may be fairly easy to obtain if big X-Ray walks.

What type of glass is in a NuArc vacuum table? I have a working small one. The glass is thin and looks like it bends under vacuum. But doesn't crack.

This won't get built in a week, more like a winter. Brrrrr.

Input appreciated.

Jim Jones
5-Sep-2016, 05:52
From one hair brained shade tree mechanic to another: Don't rely on the expensive professionally engineered light tables for inspiration. A large budget makes up for little imagination. You need to shine UV light through a negative pressed tightly against Light Sensitive Material. Traditionally the negative is pressed between a glass sheet and the LSM. with either vacuum or some soft material providing even pressure. You can also use flexible film (which can be the negative itself) instead of glass and apply vacuum to compress the negative and LSM against a rigid opaque surface. Since glass is cheap and durable, it might be the best choice. The light source can be above it and the LSM below, or this can be inverted. The glass can be stationary and the negative and LSM moved into contact with it, or the LSM can be stationary and the glass moved down upon it as in many commercial light tables. Since the glass is heavy and expensive, the former may be more practical for a one-off system. If this is done, vacuum may be more practical than weight for intimate contact between negative and LSM. Alternately, the negative and LSM can be pressed against the glass hydrodynamically or mechanically. The latter has been used to exert much pressure through leverage as in the printing presses of centuries ago. However, glass is not really a solid. It behaves rather like an extremely viscous fluid, and slowly deforms under pressure. This is immaterial when a vacuum presses negative against LSM, but can become significant when external pressure is applied. When I worked in Graphic arts, a story was still circulating of the glass in a large printing frame in the privacy of a darkroom breaking under the pressure of two rather active bodies.

Randy Moe
5-Sep-2016, 06:13
Last line got me laughing.

Good advice. I like the 'upside down' idea, especially if I add vacuum. Light under glass eliminates lots of Rube Goldberg Rigging.

Back to the sketch pad.


Christopher Barrett
5-Sep-2016, 06:45
Last month I was at this platinum printing workshop in New Hampshire. The UV exposure unit had glass that was maybe 40x60. I don't know exactly, but it was big. The neg and paper sat on a foam base. You could easily see that the neg and paper were not in good contact until the vacuum was turned on. Just sayin' I wouldn't rely on the weight of the glass to create good registration. Vacuum system's are easy to build, cheap and DIY info is abundant on the web. You could build it in while you're designing the thing.

And the response above makes me realize that the digital kids will never have memories of sex in the darkroom. :(

Pere Casals
5-Sep-2016, 07:30
Does tempered glass pass UV well?

Normally, commercial glass for general applications blocks UV very well (because SiO2 and other). Sun tanning of human skin it's almost not activated if sun rays passed through a window glass at home. Also it happens with car's windows. This is useful to protect people and objects like plastics, fornitures, fabric...

Ordinary window glass passes about 90% of the light above 350 nm, but blocks over 90% of the light below 300 nm (UV )

>> A twice thick window glass, (if not tinted) has near the same VIS transmitance as light is lost in the air-glass-air interfaces, but not much inside the glass itself.

>> A twice thick window glass has way lower UV transmitance, as UV opacity (density...) is due to absorbance inside the glass, like with a VIS tinted glass.

Is 1/2" glass going to cause distortion or softness.

I think not at all, film emulsion is in direct contact to paper. I contact print 8x10 placing paper and film in the middle of 2 sheets of 8mm glass, after inspecting paper with a 20x magnifier I see no sharpness loss from the negative.

If you make a contact print of an USAF 1951 glass slide target on Ilford Multigrade IV RC I guess you can expect to see around Group 5-Element 5 bars with a magnifier, this is some 50Lp/mm. If you place even a 1" glass sheet over the target slide to make the contact print you'll also see those 50 Lp/mm. The limit is from the paper resolution and, of course, from the sharpness the negative has.

BTW... nice project.


PD: see UviClear™ glass , but you can find cheaper (alibaba...) "ultraviolet transparent glass"

Pere Casals
5-Sep-2016, 09:29
Not planning on vacuum for now...

A very flat base can be used, marble stones can be found really flat.

Randy Moe
5-Sep-2016, 10:00
I am now thinking light under fixed glass and use a vacuum bag 'type' top pressure plate.

Almost this idea. http://www.bondlineproductscorp.com/mobile-rvb-carts.html

And this idea, but modified for our uses. I would use a power pump. https://youtu.be/IjQe9p1LBCk?t=335

A comment on another youtube was this, which makes sense and makes it easy to seal bag to glass.

"As a boatbuilder i have been bagging for some years now in and on odd shapes , Yes each one has its own twists and problems ( convex-concave 20meters??? the list is there) But for flat table work why do make it so hard for yourself with the glues etc??
(9 times out 10) i use 2 sheets of heavy plastic sheet to fit the job and seal it with brown packing tape,,, job done, Also i lay a plastic net ontop of the work to allow the air to travel ( inside the bag) try,, good fun, regards rik"

Jim Jones
5-Sep-2016, 10:51
If the negative and light sensitive material are in intimate contact, very little detail will be lost. If they are not in intimate contact, detail loss depends largely on the size and distance of the light source. In contact printing on diazo film which requires long UV exposures, we used a one foot square light source of 10 kw of metal halide bulbs suspended maybe 30 inches above the printing frame. This set-up demanded intimate contact between negative and diazo film when duping half-tone negatives. It also demanded dodging the center of the frame to equalize exposure for critical duping. Half-tone negatives could be contacted with a small incandescent lamp in the ceiling onto more sensitive graphic arts film before the printing frame drew film and negative into perfectly close contact.

Randy Moe
5-Sep-2016, 11:13
And like happens with huge enlargers, I got an offer of a large old vac table, so it goes...

Gotta see if I can fit it in here.

Funny how this 'huge' Saltsman has a footprint only 1" wider than a Beseler 4X5 MX.

Kinda like everybody here complains most 8X10 cameras are too heavy.