Transparency ‘etching’

Hello all, hope you’re keeping well and safe wherever you are!

It seems the phenomenon of visible ‘etching’ on the emulsion side of Kodachrome slides is well-known. However, nowhere have I seen anything to suggest that this is the norm with standard E6 processed films. I was thus rather curious to notice on my latest batch of home-processed colour slides that a similar effect is present. This doesn’t seem to be limited to a particular emulsion; it appears on E100, Velvia 50 and Velvia 100 alike, in 135 and medium format transparencies. What made me doubly curious was the fact that it’s also present upon my b/w reversal slides (for what it’s worth, Delta 100 with a permanganate bleach).

While this isn’t a problem at all (if anything, I find it rather pleasing!), I would be interested to find out why this occurs, and what - chemically speaking - is the cause. Which particular aspect of the process affects the emulsion in this way? Why does it react as it does? I look forward to learning something new!

I recall an article in a photography magazine many years ago, where the author had somehow photographed or copied the "etched" image on the emulsion of Kodachrome slides, to produce very pleasing "litho" type prints. I've no idea of the reference now, but might be worth some research for anyone interested ?

Kodachrome transparencies show raised relief because the emulsion layers are very thin (compared to E6) and because the dyes are added at the time of development, rather than at time of manufacture (like with E6).
If you are seeing raised relief with E6, I would be concerned.

I don't think there's cause for concern. I've noticed this as well on all my slides (both lab and home developed), but it is _much_ less prominent than what is seen on Kodachrome. I don't think you'd notice it unless you were looking for it. The relief is most noticeable when the emulsion is swollen after processing.

I've always thought that it must be due to some image-wise cross-linking of the gelatin during some step, but I can't really think what step that would be off the top of my head. This is the premise of dye transfer/technicolor: a tanning developer cross-links the gelatin (the oxidized developer molecule reacts with two adjacent polymer chains of gelatin, bridging them together). This cross linking raises the glass transition temperature of the gelatin, i.e. it "hardens" it. Then the unhardened gelatin is washed away in warm water. In this way, you get a relief image where the thickness of the gelatin is in proportion to the amount of exposure. Now whether a similar process is responsible for the relief image seen here , I don't know It would be rather surprising to me, since modern films are already extremely well hardened from the factory.

It would be really neat to run the film through an AFM (atomic force microscope) and see if the relief "image" is a positive or a negative. Unfortunately, the use of an AFM (at my school anyway) is rather expensive for curiosity's sake.

MattKing said:

Kodachrome transparencies show raised relief because the emulsion layers are very thin (compared to E6) and because the dyes are added at the time of development, rather than at time of manufacture (like with E6).
If you are seeing raised relief with E6, I would be concerned.

Click to expand...

Having read this I checked some of the last E^ I used back around 2005 and the 'relief' is there on slides of Kodak E6 so it is normal on all slides I would recon.