Dilution ratio misread as a fraction

Lifted from concentration and dilution, which sets out the notation this course uses and why it refuses the one that can be read two ways.

What you see

A working solution about ten per cent away from the strength intended, in either direction. On the negative that is a small, consistent shift in density and contrast that survives every attempt to blame the film, the exposure or the thermometer, because none of those is wrong.

The tell is reproducibility. A misread ratio is a systematic error — it happens the same way every time you mix from the same sheet — so your negatives are all slightly wrong in the same direction and none of them is wrong by accident.

Likely causes

1. A colon read as a ratio to the total. “1 in 10” almost everywhere outside photography means one part made up to ten; a photographic sheet writing “1:3” usually means one part of stock and three of water.
2. A sheet writing 1+10 transcribed into a notebook as 1:10 and then made up to ten parts rather than eleven.
3. A formula taken from a secondary source, a forum post or a memory, where the notation was changed on the way.

What is happening: the chemistry and physics

Nothing chemical. This is arithmetic, and the arithmetic is worth doing once so that the size of the error is not a matter of opinion. Take a stock at 100 g/L.

• 1+9 read correctly, one part in ten of finished solution, gives 10.0 g/L.
• Read as one part in nine total, it gives 100 divided by 9, which is 11.1 g/L — about 11 per cent too strong.
• A sheet meaning 1+10, one in eleven, read as “1:10” meaning one in ten, gives 10.0 g/L where 9.09 g/L was intended — about 10 per cent too strong.

🧮Ten per cent is not a rounding error

ILFORD’s own temperature compensation chart makes 1 °C of developer temperature worth about 9 per cent of development time, and darkroom workers take great trouble over one degree. A misread ratio is the same size of change, arrived at without noticing, and it does not announce itself the way a warm tank does.

Diagnostic questions

• Does the sheet give volumes anywhere? This is the question that settles it, because a table cannot be ambiguous. ILFORD’s ILFOTEC LC29 sheet says a 600 ml tank at 1+9 takes 60 ml of concentrate and 540 ml of water, and sixty plus five hundred and forty is six hundred. Kodak’s HC-110 sheet labels 75 ml of stock with 225 ml of water as “1:3”. Work from the table, not from the notation.
• Recompute the working concentration both ways and see which one matches the volumes you actually measured.
• Is your result about ten per cent from where you expected, rather than a factor of two? A factor of two is a different mistake.
• Did the notation change between the manufacturer’s sheet and your notebook? One manufacturer is not even consistent with itself — ILFORD gives its wetting agent as “1+200” on one sheet and “(1:200)” on another for the same product in the same role.

Corrective action

Discard the working solution and remake it from the manufacturer’s own volumes. There is no way to correct a bath in the tray, because a bath that is 11 per cent strong cannot be brought back by adding water without knowing exactly how much is there and what it started as.

If negatives have already been developed in it, they are not spoiled, but they are not comparable with anything else you have made. Record the true dilution against them so that the series stays honest.

Prevention

Write 1+9 and never 1:9. Say so the first time on any page, sheet or notebook entry that uses it - one volume of stock plus nine volumes of water, giving ten volumes of working solution. The plus sign says what to add. The colon says a ratio and leaves the reader to guess what it is a ratio to, and the guess is worth about ten per cent of the developer either way.

Where a manufacturer publishes a table of volumes, copy the volumes into the notebook rather than the ratio.