For its first 160 years, photography was based on silver. Effectively all camera negatives have to be made of this metal because only silver halides are fast enough to record analog chemical images "instantaneously"—or even in a couple of minutes. But printing from the negative need not be speedy, so the door is open to several slower photochemical processes for printmaking. Its no great disadvantage to use strong light sources and long exposures, especially when the negative is large enough for contact printing, which allows much more throughput of light than an enlarger. A century ago, besides silver, many permanent prints were being made in platinum, palladium, and pigments such as Prussian blue.
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