Platinum print contrast test

Platinum print Contrast test.

This note was written based on this PDF file, which is generated by AI.

It is told that the ratio of chlorate decides the contract of Platinum print. However, I have never had a chance to check it, and I’ve been wondering how much the contrast can be changed. In fact, the contrast of my platinum prints cannot be completely stable. Even though I print same digital negative with same mix in same day, the contrast of these prints can be different. So, I’ve not been sure how much I can compare the contrast by the ratio of chlorate with my test prints. Anyway, this month, finally I had a chance to try this. The results are not 100% convincing, but some analysis makes sense to me. I used AI to check all the number and asked AI to analyze my findings.

I have prepared 5 coating papers in 5 different grades which are Grade0, Grade2, Grade5, Grade7 and Grade9. Grade 0 is 0% chlorate. Grade 2, 5, 7 and 9 are 25%, 50%, 75% and 100% chlorate. Please see Appendix to mixing details for each Grade. I printed same digital negative for each grade and compare them. Of course I wanted to adjust the curve after printing them. I prepared another 5 coating papers with the new curve and printed them on 8/10. I got another adjusted curve again and printed them on 8/19. So totally I had 3 chlorate series for each curve. Now I have 15 test samples and additional grade 5 test samples.

Here are the results.

Figure 1 — The two ends of the dial and the middle, all from the same negative and the same day (Series 3). 0%, 50% and 100% chlorate. The middle sheet, not the last one, has the lightest mid-tones.

The grade dial works, but only over its bottom half

In short, 50% and 75% chlorate has the most contrast as far as I measure. It is the left graph of Figure 2. The y-axis is the line slope on the mid-tone range. Each curve can has the most contrast at 50% or 75% chlorate. 100% of series 2 (the red line) is very low, which means the slope of mid-tone is flatter.

The second graph, reading the mid-tone — the density the print makes where the negative is at its 50% step — the three series say the same thing. Going from 0% chlorate to 50% chlorate, the print gets lighter in the middle by 0.28, 0.26 and 0.30 density. That is about 0.14 for each quarter of the dial, and it is repeatable across three different negatives.

Above 50% it goes backwards. From 50% to 100% the mid-tone comes back +0.24, +0.22 and +0.09. So Grade 9 is not the hardest print I made. Grade 5 is. In all three series the turning point is at exactly the same place.

Figure 2 — Three ways of asking the same question. Left: contrast, the gradient through the mid-tones — this is the actual contrast measurement. Middle: where the mid-tone sits, which is a point on the curve and not a slope. Right: the maximum density. All three panels span the same 0.85 density, so the vertical scales are comparable.

I did not expect this, and I don’t think it means the chlorate stops working. Every one of these sheets got the same exposure — full lamp, 12 minutes, which is the setting for the max black.

What changes is the shape, not the range

The maximum density barely moves. Across all fifteen sheets it is 1.22, and every single sheet sits between 1.16 and 1.29. Paper white is paper white. So chlorate is not giving me a longer scale — the two ends stay where they are and the middle slides between them.

Figure 3 — One contrast series, five chlorate mixes, printed through the same correction curve. The 0% sheet climbs fast and then flattens; more chlorate flattens the start and steepens the middle.

Looking at the slope tells the same story from the other side. In the highlights (negative steps 0–30%) the gradient falls from 2.6 at 0% chlorate to 1.8 at 50%. In the mid-tones (steps 30–70%) it rises from 0.58 to 0.85. The chlorate is holding the highlights back and handing that gradient to the middle of the scale. That is what I actually see in the prints: at 50% the sky and the water open up, and the shadows close down.

Figure 4 — One picture from the test target, printed across the whole series. Same negative, same exposure, same developer; only the chlorate changes. This crop is mostly high values, which keep opening up all the way to 100% — the reversal in Figure 2 is in the mid-tones.

It is a smaller dial than the things I am not controlling

This is the part I find most useful, and it is the answer to the doubt I started with.

• Changing only the correction curve, same day, same mix: up to 0.13 in the mid-tone.
• The same mix and the same curve printed a week later: +0.13 and +0.02, both times denser.
• One fresh batch of ammonium citrate developer: 0.17 lighter than the aged batch it replaced.

Against that, one full quarter-turn of the grade dial is 0.14. A week of developer ageing moves my print as much as jumping two grades. So my feeling that my platinum prints are not stable was right, and now I have a number for it. If I want to compare grades honestly, the sheets have to be printed the same day, on the same negative, with developer of the same age — which is what each of these three series is.

How I measured it

Every sheet is Bergger COT320, solution C (platinum) held at 0.45 on all of them, so the only thing changing across a series is how much of the A+B total is the chlorate solution B. All 21 sheets: full lamp 12 minutes, ammonium citrate 5 minutes.

Density is read from the Epson V850 16-bit raw file — gamma 1.0, no curve — as −log₁₀(pixel ÷ that sheet’s own bare paper margin). The scanner field is flat to 0.002 along the wedge, and the repeat floor of the measurement is 0.002, so everything above that is real. Two sheets from 8/17 had coating faults and are not in the numbers. One thing I still assume rather than know is that the scanner is truly linear — nothing in these scans has a known density.

Appendix: Mixing details by grade