Letterpress printing pressure per square inch

Has anybody here ever seen an estimate of the average area of ink on a square inch of average newspaper text? I am trying to work out how many PSI are applied to the face of the letter by a typical hand press able to apply about 18 PSI per square inch of platen, but that is a solid square inch.


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I think if you scan something and then load it into certain of the graphics programs (which a graphic designer or an up-to-date letterpress person would know), that those programs will give you the ink coverage of the job. You could scan some newspaper text.

Good luck and let us know how it works out.


I took a square inch of text from a newspaper article and blew it up and overlaid it with a grid and estimated what percent of each grid square is black on average, and arrived at an estimate of 17% of a square inch of unleaded ordinary roman text would be black. I calculated that each square inch of solid area receives 18psi of platen pressure. If the pressure is concentrated into the areas of contact only, at that rate each letter receives about 100psi on average. Seems a bit high, but …


I think you went about it the right way.

I keep in mind too, that ultimately a press is capable of exerting a great deal of pressure. Normally we don’t require it to do that, and we keep it from doing that by reducing the packing when necessary. (This would apply only to presses which have the distance from bed to platen [or cylinder] when on impression, set and fixed in advance. This includes hand presses which are pulled against a stop to keep the impression uniform. It would not apply to small lever presses where the operator can vary the impression pressure).

If we want to increase the pressure, we increase the packing (either overall or in selected places on the forme), and vice versa. Thus, even though a press is capable of exerting 18 psi, as you say, we usually have the packing reduced so that the press only exerts enough pressure to get good print quality.

So when you say 100 psi for text seems a bit high, it very well may be high.

That’s my 2 cents worth…..

Geoffrey, good point. The pressure I calculated is pretty much the maximum available under normal circumstances, and many surviving iron hand presses are evidence that such a limit can be exceeded by pressmen anxious to “make an impression” of something at or beyond the envelope of limitations. The best quality is achieved not by “pushing the envelope” but by careful preparation and exacting standards.


Each press is probably very old, so I would imagine the pressures between each to vary. Asking for a PSI of “18” rather than “15-20” seems a bit optimistic to me.
How are you measuring the empty pressure of the press? IE: it’s pressure output for a given setting with a given set of MR sheets, over say 1 square inches? 10 square inches?
Many years ago, I ran across a clear film that, when hit with pressure, would turn pink. The darkness of the color indicated more pressure. I think the color could be compared to a card, giving an approximate PSI. They tried to sell it as a way to even die pressure. It did not take off really as a number of sheets were needed during the MR process, and a trained eye could skip that step anyway.