Measuring a clearing time

Purpose

To turn one scrap of film into two numbers: how long to fix today, and whether this bath has anything left. Both are measurements of your bath rather than figures about somebody else’s.

When to use it

Whenever the bath is mixed, before anything you care about, on the schedule your capacity log sets — Part XI suggests every quarter of the published capacity — and any time a negative comes out of the tank milky.

The test needs undeveloped film. It is not run on paper, and a paper bath’s condition is judged by the film test and the log rather than by looking at a print.

Before you start

You should already have the fresh clearing time of this bath, measured on the day it was mixed and written on its label by the labelling procedure. Without that figure the test still gives you a fixing time and it cannot give you a retirement decision.

On the bench: three scraps of undeveloped film of the stock you actually use; the bath at its working temperature; a thermometer; a timer; a viewing station you will use every time — the same lamp, the same background, the same distance; the capacity log; the silver-bearing waste container. The scraps are sacrificial, so the whole test runs in room light.

Procedure

1. Bring the bath to its working temperature, take the reading and write it down. A clearing time without its temperature is not comparable with the next one.
2. Lay one scrap emulsion side up and put a single drop of working-strength fixer on it.
3. Leave the drop until the emulsion under it is a clear spot, about 30 to 60 seconds. That spot is your reference for what fully cleared film looks like on this stock.
4. Immerse the whole scrap in the bath and start the timer.
5. Agitate exactly as you will agitate every time you run this test — one gentle movement at a fixed interval, written down once and never varied.
6. Stop the timer when the whole piece matches the reference spot, judged at the viewing station and not at the window.
7. Repeat steps 2 to 6 twice more on fresh scraps. Three readings, because two cannot tell you your own repeatability.
8. Take the mean of the three. Today’s fixing time is twice it.
9. Compare the mean with the fresh figure on the label. Retire the bath when today’s clearing time exceeds twice the fresh one, whatever the sheet count says.
10. Cross-check against the maker’s published fixing band for your film. A clearing time that puts the fixing time outside that band is telling you something about the bath rather than about the film.
11. Add the area of the three scraps to the bath’s running total, and write the line in the log.
12. Put the scraps and the rinse into the silver-bearing container by its own procedure, and read the disposal caveat before any of it leaves the house.

How you know it worked

• Three readings, and they agree with one another closely enough that you would accept any of them.
• The reference spot was made on the same stock as the piece being timed.
• The temperature is written beside the readings.
• The bath’s label and the log now carry today’s figure, its date and the running total.
• A decision came out of it: fix for this long, or retire this bath.

If a step fails

Step 3, no clear spot forms under the drop. The bath is exhausted, or it is not fixer. Check the label; if the bath is what it says it is, retire it. There is no fixing time to be had from a bath that cannot clear a spot in a minute.

Step 6, you cannot decide when it matches. That judgement is the instrument, so improve the instrument rather than the guess: dry your hands, use a dark background and transmitted light, and lay the piece beside a scrap that has been in the bath for five minutes as an unambiguous cleared end.

Step 7, the three readings scatter widely. Agitation or temperature moved between them. Redo all three with the timer, the thermometer and the same movement, and use the redone set.

Step 9, the bath crosses the retirement criterion mid-session. Stop loading it. Anything already in it is fixed in a fresh bath for twice a newly measured clearing time before it is washed; work already washed is checked by the residual tests rather than assumed sound.

Step 9, there is no fresh figure on the label. You have a fixing time and no retirement decision. Record the reading as the fresh figure for a bath of unknown age, mark the label age unknown, and use the bath only for work you are willing to lose until it has been replaced.

Step 10, the fixing time lands well outside the published band. Look for the ordinary causes first: a mis-mixed dilution, a coupon from a different film, or a bath at the wrong temperature. Only when those are excluded is it a statement about the bath’s age.

What to record

One line in the capacity log: the date; the bath, its dilution and its mixing date; the temperature; the three readings and their mean; the fixing time taken from it; the running total after the test; and the decision — carry on, or retire. On the day a bath is mixed, the same line is what gets copied onto the label as the fresh clearing time, and everything above depends on somebody having written it.