What is solarization in photography?

It's when whites are turned into darks. A solarized image has to be in the developer for a short time until the image could be a little seen and then dried and exposed again under the enlarger.

I've never heard of doing it this way and can't see how it would ever work for several reasons I won't go in to. The method, as I learned it and always taught it is as follows: with the negative set up as normal, under-expose the paper slightly and remove the negative from the carrier. With the carrier back in the enlarger to block excessive light leakage from the carrier slot, remove the easel and move the print to the developer tray. When the image first begins to appear, place the print in the water bath for about 10 seconds to arrest development. Put the wash tray under the enlarger lens with the print still in it and with the lens aperture stopped down to F16, turn on the enlarger for about 2 seconds and blast the print with the enlarger light. Place the print back into the developer tray and complete the print processing as usual. The effect for a given print can be varied by experimenting with the amount of initial image exposure, initial development and subsequent exposure to the bare light. Alternatively, a 20 watt light source from about 4 feet can be tried instead of the enlarger. The second enlarger exposure could be dimmed if necessary by using a strip of partly exposed and developed film or shortening the time. The exposure can be increased by increasing the aperture. The technique s properly known as the Sabattier effect, and provides better results with high contrast paper. The "silver" lines that are often created between the original dark and "new" dark areas are Mackie lines. They happen because a boundry is created between the first partly developed dark area and the second dark exposure area. Not much development takes place along the boundary line.