Sunday, 23 September 2012

A fixing question.

This is the second most important part of the process after development. Proper fixation ensures the longevity of your prints and negatives. Fix works by removing the unused silver bromide particles from the film or paper. If not done properly, over time they turn black ruining the image.

The way the fix works on the emulsion is to chemically convert the remaining silver bromide into complex argentothiosulfate an insoluble  and unstable compound which after a few seconds can be seen on the film  as a milkiness (not visible on prints) this should be allowed to continue until it has disappeared otherwise the negatives will turn black. As the process continues the fresh hypo from agitation turns the insoluble into soluble sodium  argentothiosulfate which can be washed away in water. The fixing of the image is quite quick, it is turning the by products into a water soluble compound that takes the time.

The darkroom practitioner needs to be carefully when choosing a fix as some cause staining when being used with paper. There are three types: alkali, plain, and acid.

         Alkali are the most modern and efficient with today’s emulsions. They are the easiest to wash out of papers and cannot be over fixed.

          Plain is a mix of Hypo (thiosulfate) and water that should be used as part of a two bath system, then only as a second one as it can cause staining and other problems.

         Acid fixes are known to be quick and should be timed carefully as they can cause bleaching. Hence the reference rapid.

Some fixes come with hardening and should not be used if you are thinking of toning prints. Otherwise it is down to personal choice.

Film Fixing.

A lot of people ask how long will the fix stay fresh and how many rolls it will process. I mix up  1000 ml or 1200 ml  depending on how many rolls of 35mm and or 120 format I think will be processed at the same time. ( I  use a different fix for paper) I keep the fix until I have developed a mix batch of between twelve and fourteen rolls. Which can take some months to achieve.

The film has a milky look after one minute in the fix
There is a more scientific way of being sure what the limits of the fix are for yourself. It can only be done with a fresh batch of fix. After the film has been in the fix for a minute you open the process tank, checking to see if the milkiness has cleared. If it has then give the film a further three minutes this will also establish the time the film needs to be fixed for. You will need to do this check  with subsequent films until the clearing time is double what it was when the fix was fresh. Having kept a check on how many films you have processed this will give you a safe number per litre for the future, hence removing the need to check each time. There are other method for checking fix life which will be posted later.

Paper fixing.

There are two methods to choose from.

         Two bath method. This where two lots of fix are made up and set side by side. The first bath does all the work, with the second bath removing any argentothiosulfate that have not  been converted to sodium argentothiosulfate in the first place, making for a more complete final wash. Once the first bath is exhausted (which you will need to test for), the second bath becomes the first and a new one is mixed for the second. When the second now first bath is exhausted both should be ditched and two fresh fixes made up.

I use this method occasionally If I'm toning. To get round all the testing you need to know how many prints you can get out of the fresh  bath and the partly used second, for each of the sizes you print most often. It is a lot of work to start with.

         Single bath. Is a fast acting concentrated fix that takes one minute to do the job. This method needs to be timed exactly; to leave it longer will negate  its advantages. Most of the fixing is done in the first fifteen seconds, to leave it longer than one minute will allow complex compounds to build up making them difficult to wash out.

I use this method most of the time partly because I do not have a lot of time to spend when printing and the other is I do not do a lot of toning.