PE, I'm not doubting the wisdom of your respected colleagues, my concern was that your proposed mechanism of the paper being "
changed on a micro scale" (due to electric currents in the metallic silver inducing heating) has no theoretical basis. I believe that the heating effects are solely due to the water (and any other polar molecules in the paper) heating up.
I think that a good reason not to dry prints in a microwave is that the uneven heating effects would heat some areas of the paper to excessively high temps. I tried drying an RC print the other day, and sure enough the corners curled up and these curled bits took on a more glossy appearance where I suspect some areas had melted. The problem could be reduced by lowering the power, but even so, some bits of the print would still receive a greater heating effect than other bits.
The rest of my reply gets into the details.
Photo Engineer said:
Peter, although an individual silver halide crystal in a B&W paper may be small, on the order of 0.2microns on a side assuming a cube of AgCl, the filament that is formed may indeed be quite long. This is assuming filimentary silver formation during development as opposed to tabular.
It is the two dimensional 'length' of the filament that is important here. As an example, if I take a telescopic FM radio antenna say 1m long (assume it is very thin for the sake of discussion), and crumple the antenna up into a small cube with a side of only 1mm, then the antenna will be useless if we want it to receive the intended FM radio waves.
Photo Engineer said:
But, as you say, the operative word is 'optimally' meaning that the highest statistical probability is if the wavelength is as you say, but that does not rule out induced current in shorter or longer filaments. (does it?)
Technically it doesn't rule it out, but the current's magnitude will be
SO small as to probably be but a few more electrons above the noise floor ! I could go and look up the maths, but we'd be talking pico, no, maybe femto-amps for sure!
The wavelengths emitted by the magnetron oscillator will have a very narrow gaussian distribution centred around the centre frequency of f=2450MHz. This is the idea behind an oscillator - they have high Q's and thus low bandwidth (i.e. spectral spread). FWIW, the frequency for which 0.2microns is a 1/4 wavelength is f=2500GHz. This is 3 orders of magnitude higher than the microwave's frequency of operation.
Now assuming an object is 'the right size' (has at least one of its dimensions approximately a few cm) then :
Metallic (i.e. conductive) objects in a microwave heat up because the electromagnetic field induces a current of electron flow which dissipates heat due to ohmic (resistive) losses.
Non conductive objects that contain polar molecules (e.g. H2O) in a microwave, absorb energy (heat up) as a result of dielectric heating.
Photo Engineer said:
There are ample photomicrographs of the filaments that form in the literature that I don't feel I have to post one here, but I do believe that they can approach a considerable length, but whether they can react with the shorter wave microwaves IDK. I doubt that anyone has 'unravelled' one of the filaments that form to actually measure its length.
'shorter' doesn't really make sense here. I assume that the filament length (if stretched out) will still be much shorter than 3cm (=1/4 of the microwave's wavelength). OTOH, if you meant there are sufficiently different length microwaves in the oven to be significant at the filament's length, then that isn't the case.
Photo Engineer said:
Not even color products which contain no silver when finally processed. (This latter was interesting as I thought of it while writing this reply)
I think the relevance of this latter point supports my notion that it is the water that is heating things up and not the silver.
Photo Engineer said:
It may just be that it has nothing to do with the silver itself, but rather a vesicular effect from the micro bubbles formed if you boil water in your wet print causing some sort of translucency. There are many imponderables here that were brought to my attention those many years ago by some very respected fellow engineers with years of experience.
Quite possibly localised heating could cause this, the object would need non-symmetrical or 'pointy' sections on it for this to happen. This is the case with the corners of the paper.
Photo Engineer said:
An added comment. IIRC, it was shown somewhere that microwaving unprocessed photo products had an effect on the latent image. If true, I wonder why?
My guess is that this is resulting from a weak dielectric heating effect.
regards
Peter