- 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 :

 

Water  700 cc, Ascorbic acid-(Vitamin C)25g ,

Sodium carbonate anhydrous50g,

 Sodium hydroxide 20g,

 Metol 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.

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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.

 

Bleaching

A simple successful bleach is  comes from using :

Ferric EDTA   40g

Potassium Bromide 70 g

20g Sodium hydrogen sulfate crystals( or 20cc  acetic acid)

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 is dry.

Queries :  jeff-at-biotech.cam.ac.uk    or  tel:01223  334152