Adorama Solarol Solarizing Print Developer for Black & White Prints, 6.4oz Can Makes 2 Qts.

Print development with Solarol provides a new dimension of image control through the use of the Sabattier effect, commonly called "solarization". The effect is produced by re-exposing the print to white light while it is in the developer. With other developers, this procedure gives a dark, muddy picture, but with Solarol, contrast is maintained, sharp edge effects appear, and a beautiful surreal image is obtained. The results are predictable and repeatable, and you control the process just as in conventional printing. Makes 2 quarts of working solution.

The tray life of Solarol equals that of the best of print developers, and it keeps for over two months in a well stopped bottle.
With extended use, its developing action becomes slower, but its solarizing ability actually improves!
It should be discarded when it is too exhausted to produce a good black with two minutes of development