Glass carrier - what type of glass?

I am building a copy system, whereas the film will lay atop a piece of glass, and photographed through the glass, like an enlarger. What is the best type of glass is used for this? Is thickness an issue? 3/8" OK? TYIA

I know the right answer is that the glass should be perfectly flat, optical glass (and probably multi-coated. But I have a confession to make: the glass in the carrier for my 10x10 enlarger is plain old single strength (1/8" or 3/32") window glass from the hardware store. The replacement glass from Durst cost more than I paid for the enlarger. My point is that it depends upon your budget, and my window glass gives results indistinguishable from prints done with glassless carriers or my Omega glass carrier.

If it is black and white copy work you may get away with window glass. Only way to find out is to try it. window glass is cheap. If it does not work you would not be out a lot of money or time trying to find optical glass. It will also tell you if your idea will work. If the work lays flat I would try thin glass.

AN glass. For an enlarger ususally made from a high quality float glass. Scientific supply houses may have this glass.

It's called anti-newton glass.

--Gary

If light is passing through the negative, then I suspect anti-newton ring glass will be the best choice. But before laying out the cash for that, try a plane old piece of 1/8th inch glass...or even 1/4 inch glass.

Actually almost all flat glass are produced with floating process giving a perfectly plane and flawless surface. You can also obtain, for a very little extra cost, ultrawhite or extraclear float glass which has no greenish cast since it contains no iron oxide. This has excellent optical quality. Keep in mind we are speaking about excellent but normal flat glass, nothing to do with anti newton or multicoated ones but I tell you this: here in Italy I can buy 5 mm float glass for USD 18/sqm ( one square meter $ 18 ) and swiss made 6 mm ultrawhite glass for USD 39/sqm, I think it is worth try.
Ciao.

Thanks for the responses, I was too thought AN glass was the safest bet.... does the fact its chrome film change anything? I assume many of you use negative as a generic term, meaning chrome or neg film?

Does your copy work involve contact or projection? This can make a difference, but in
either case Newton rings are a real risk, since transparency film will exaggerate the contrast. Original transparencies also tend to be slick and create rings on the base side. I make duplicate trans and negs rather frequently by both methods and can't imagine doing this without high-quality A/N glass. For some B&W films optically-coated
glass is OK, but in my experience, never for color. But I live in a foggy climate where
Newton-rings are the curse of the darkroom. Otherwise you could try an anti-newton
spray, available from scanner suppliers. I believe Focal Point sells AN glass at a reasonable price.

I use normal glass in my 10x10 enlarger, and I have never had any visible problems showing up in the print. I do up 30x40 and 20x50 prints with no problems. I would highly recommend that you do not us anti-newton glass because it is like nonglare glass with one surface etched that comes in contact with the film. This will soften the image of the film you are trying to photograph.