I am providing information on three separate experimental techniques, as there may be some readers who would prefer to try producing a print that appears different than a normal photograph. The first of these techniques pertains to negatives and is called "reticulation" and the second pertains to prints and is called "solarization". The third technique is double-exposure, using two negatives, sandwiched together. There are other techniques, but as far as I am concerned, these are the two that are most interesting. These techniques can produce very interesting prints and you should at least attempt solarization since it is by far the easiest to experiment with. I have seen prints made from reticulated negatives, but have not actually tried that technique myself.

Reticulation

This is a technique which will crack and distort the emulsion of a negative or roll of negatives. When the negaive is then printed you can get interesting effects. Reticulation will give inconsistent results and you will not be able to duplicate the effects with subsequent processing.

The process usually involves developing the negatives in very hot solution - approximately 100 degrees fahrenheit. Unfortunately, film manufacturers do not publish development data for processing at such high temperatures.
So instead, I would recommend that you develop the film normally and then once the development is finished, pour out the developer and fill the processing tank with hot stop bath (approx 100 F or 38 C) (at the correct concentration). Now pour out the stop bath and immediately fill the tank with an ice cold water rinse. Since the emulsion is still soft, this rapid temperature change should crack and warp the emulsion. Finish the processing with fixer and wash water as you normally would.
When the negatives have dried and you attempt to print them, the resulting prints will yield various degrees of visual cracking and, or distortion.

Solarization

This technique is used on a partially exposed, undeveloped photographic print. Commonly referred to in photographic circles as "solarization", this process is actually a variation of the "Sabatier Effect".

I have tried solarization many times and have achieved some pleasing results. This technique, if suttle enough can enhance certain subject matter or sometimes give a surreal look to a variety of scenes. Your subject matter can be most anything. The finished print will have the effect of a part negative, part positive look. The midtones and shadows will retain a positive look, while the highlights will be darkened with a white outline (negative appearance).

Solarization can be achieved by setting the exposure to one-quarter to one-half stop of the correct exposure that would be required for a normal print. Expose the print and then process for about 50 seconds in fresh developer.

The light source is normally a frosted light bulb with a wattage anywhere from 7.5 watts to greater than 100 watts. Position the light bulb about 3 or 4 feet above the development tray and flash it on for 1 to 10 seconds, depending on the wattage of the bulb. Continue development for another 60 to 90 seconds, or until you like the effect.

As this technique is experimental, try adjusting your original exposure time, the initial processing time and the wattage of the light bulb and its distance from the development tray.

Your finished print can also be toned for an even more dramatic result.
See: Print Toning

Double Exposure

With a little patience, you can achieve interesting double exposure effects in the darkroom. You can sandwich two negatives together in the enlargers negative carrier. Align both negatives with their emulsion (dull side) facing down toward the easel.

This technique does not work well if the negatives are not of the same density. Achieving a suitable filter pack and exposure time can be difficult, so use test strips to determine the exposure.

To make the most of this technique, use a little thought when you make the exposure using your camera and try and previsualize the final result.