Notes on Developing and safelighting
Before exposure, plates greatly benefit from a presoak for 30 sec in 7% TEA (Triethanolamine) under safelight. Followed by squeegeeing off excess with
a windscreen wiper blade. This can increase their photosensivity considerably. If you leave the TEA in the emulsion and dry
the plate in air before exposing , you must allow at least 30 minutes (preferably even more) to allow plate to stabilize and
stop creeping after this treatment. TEA treatment gives the brightest results and shifts the replay wavelength from red to
yellow/green if it is not washed out. Even if you dont want your plates pre-swollen because you want your picture to be orange-red
, you still gain in photosensitivity after you wash out the TEA before exposing.
A good developer for both transmission and reflection holograms is :
cc, Ascorbic acid-(Vitamin C)25g ,
Sodium carbonate anhydrous50g,
4g . Make up to 1 litre
This should be used in a plastic dish with an identical dish floating on
the top so that there is a minimal air gap. If you use it like this the same
developer can be used again and again for many days otherwise this developer left in an open dish is more vulnerable to oxidation
from the air than standard developers because it does not contain sulfite ions.. So the time the developer spends exposed
to the air is low when you use a floating dish and it is only while immersing your plates or taking them out that it is fully
open to the air.
The exposure time should be sufficient
to enable the developer to get enough density in under a minute (optical density in centre should be around 2.0, see below).
DONT NOT BE TOO TIMID ABOUT SAFELIGHTING.
It is important to see what you are doing
and how things are developing even if a
small amount of fogging may be occurring. A little bit of fogging is OK
if you are going to bleach your holograms. For red sensitive plates I use a 10
or 15 watt green bulb as safelight about 10 ft away, (usually a bit of black tape is preferable around the join between the
glass and the metal base to stop white light leaking through.
How much development time?.- A
test method: Leave a small piece of plate in very bright light for a while and then leave it in developer for 5 mins. so that
it goes as dark as possible. Rinse and dry it. Now when you develop your holograms under safelight you should use this dark
piece of plate to look through at the safe light (it will still have some transparency). You then know that your holograms must never go as dense as that or you have gone too far on developing. .It is OK as a working principle to take your plates to no more than half that density in
their darkest areas by looking through at the safelight filament with one eye shut. (Plates continue to develop well in air
without staying in the dev. bath all the time.)
After developing, plates are rinsed under tap and dropped into the following bleach bath.
After about 20 seconds the bleach progress can be viewed under white light.
A simple successful bleach is comes
from using :
Ferric EDTA 40g
Potassium Bromide 70 g
20g Sodium hydrogen sulfate crystals( or 20cc
Dissolved up in 1litre water (tap
water is OK here).
In this bath the black silver
disappears rapidly and you can then rinse under tap.
The reflection hologram (if you have one) will appear only when the plate
Queries : jeff-at-biotech.cam.ac.uk or tel:01223 334152