Advice on enlarger problem

[winger]

I've had negs pop in my 23C during a 25 second exposure. It only happened in the winter in my old darkroom and when I was split filtering.

[fotch]

The negative is very flexible and directly in the light (heat) path. The tape will only keep if from sliding around, it cannot support the middle of the negative. That is why it still pops.

An easy way to prove this is go to a glass dealer (hardware store?) and get a piece of picture frame glass cut to fit where the negative holder goes. Then put a scratch on the glass and fill the scratch in with a marker. Or, just draw a fine line in with the marker.

Focus it and see if it goes out of focus. If it does, then its a mechanical problem, if it does not, its negative pop.

I suppose you could get 2 sheets and sandwich your negative between them, if there is enough space as another way to prove this to yourself.

[Bob Carnie]

This is a pretty good idea

The negative is very flexible and directly in the light (heat) path. The tape will only keep if from sliding around, it cannot support the middle of the negative. That is why it still pops.

An easy way to prove this is go to a glass dealer (hardware store?) and get a piece of picture frame glass cut to fit where the negative holder goes. Then put a scratch on the glass and fill the scratch in with a marker. Or, just draw a fine line in with the marker.

Focus it and see if it goes out of focus. If it does, then its a mechanical problem, if it does not, its negative pop.

I suppose you could get 2 sheets and sandwich your negative between them, if there is enough space as another way to prove this to yourself.

[Casey Kidwell]

Fotch, great idea. I cut a piece of glass to fit the enlarger and made some marks with a fine point sharpy. Tested it for 30 seconds for several intervals. Then I cursed at it for another twenty and slapped it lightly a few times. Stayed in perfect focus.

I'm not finding a glass carrier for this enlarger. Guess I'll have to make one or buy the accessory heat shield and hope it works. Someone suggested pre-heating before I expose paper which should work fine but I would prefer a more permanent fix.

What drives me crazy is that my lamphouse just doesn't get very hot. The glass I placed in it for the test was heated up for a minute and was barely warm. And the 75 watt bulb and film are separated by 2-3 condensors and VC filter. Might be the humidity change. Thanks again everyone.

[richard ide]

If you can do it; try one piece of glass on top of your negative. The negative will be held very flat because of the direction of film curl.

[Worker 11811]

Although heat is the main cause for the effect it's not really the whole problem.
Differential expansion of dissimilar materials is the culprit, behind the scenes.

The cellulose triacetate base and the gelatin/silver emulsion expand and contract at different rates when heated or cooled. The uneven expansion of the two substances is what causes the warping.

I said above, the bi-metallic strip inside a coil spring type thermometer works on the same principle. A very small difference in the expansion of the dissimilar materials inside the thermometer can cause a large movement in the needle of the thermometer.

Reducing heat will reduce the problem. And, as explained, reducing heat even by a small amount can give a comparatively large reduction in film warping.

Heat absorbing glass will help. It did make a difference on my Beseler 23C.
Getting a better negative carrier can help. Getting a glass negative carrier is best.

[tlitody]

could be negative popping, especially if enlarger type is condensor with bulb vertically above film. The diffusion enlargers with bulb off at 90degs to film aren't so bad as you only get light from mirror and not direct heat.

But another potential problem is cause focussing up rather than down. i.e. if the focus movement moves the lens stage up as its last movement it can sometimes settle back down a little. Always try and make last focus movement down so there is no downward settlement of lens stage afet focus.

[MattKing]

I'm not aware of there being a glass carrier for the Beseler 67 series enlargers.

You mentioned that there was a VC filter above the condensors. Do you have the same focus shift problems if you print without a filter?

Is the head seating correctly on top of the negative carriers?

Does this happen if you don't tape your negative in?

[fotch]

Heat absorbing glass probaly will solve your problem, however, if humimidy is also playing a part, then you may consider a De-humidifier. One other fix would be to reduce wattage of the bulb.