PROS & CONS OF MOUNTING SLIDES IN GLASS
Or the GLASS MOUNTAIN
By John Ruffell LRPS APAGB FRSA.
Some people are against using Anti-Newton glass mounts because they may reduce the definition and contrast of the slides.
I used them for many years without noticeable deterioration in the projected image. Technically it may be so, but I always use glass to protect the original slide. The amount of dirt and grease I remove when cleaning the glasses gives an indication of how much may have got onto the film. Any projector acts like a vaccum cleaner sucking in towards it fumes and dust in the air, as can be seen in the projected light. Glass mounted slides also prevent the film `popping` and thereby going in and out of focus however slightly while the image is being projected. Also with glass mounting special effects using masks are more easily achieved. So on balance I feel Anti-Newton Glass Mounting is preferable. Plain glass does cause Newton Rings i.e. the `rainbow` ring effect when the film touches the glass.
A couple of disadvantages of mounting in glass is the possibility of fungi developing and of burning the slide in the projector `gate` if it is left there too long. The fungi problem seems more one of storage than happening in use when projected quite often. If the slide magazines can be sealed in plastic gags bags, containing silica gel (obtainable from chemists), this should prevent moisture and condensation within the bag. Fungi usually shows up first as tiny black dots, and if found in time re-mounting the slide in new glasses is maybe all that is required. There are some proprietary mixtures which are said to clean the film and protect against fungi, but I haven`t tried any of these. With regard to burning, all projectors I have come across run hot, despite claims to the contrary, and if a slide is left in the gate long enough mounted in glass a hole will be burnt in it! In normal use this is no problem, and with Audio Visuals and electronic fading, projectors run much cooler than a single projector does when the bulb stays on continuously.
Some experiments were carried out with `gate` temperatures in projectors in the Midlands a few years ago, and no make or model was found to run at a satisfactory temperature. Some projectors could take an extra heat filter and that did solver the heat problem but made the projector give out considerably less light. Probems have been experienced in the single projection of glass mounted slides in Club competitions with judges commenting on them while they were being projected. You may say that the judges comments were too long any way, but that does not solve the problem! For the purpose of club competitions it seems best not to glass mount. Larger exhibitions usually prefer glass mounted slides, and here the judging takes place in seconds per side, and are usually projected at the exhibition fairly quickly or as an Audio Visual so the problem does not arise.