Image onto black slate

Believe it or not, slate is actually very sensitive to the laser. It is not easy to get mid-tones, it either etches or it doesn't. The key to getting a good engraving is prepping your image to try and eliminate the majority of the mid tones. This image originally had allot of smoke and snow that showed up in the light gray range. Playing with not only the contrast but the gamma can get you to a point where the mid tones are very subtle and subsequently the white is not overwhelming.

Transforming it into an ALMOST completely black and white image was the key

I did not treat this one with anything. Originally, I sprayed with a clear coat prior to engraving to darken the slate. I also sprayed with a clear after it was done. These layers of clear made it very difficult to see the engraved portion. I was on the verge of tossing it and trying again when I decided to take some acetone and remove all of the clear. When left with just the slate, I was very pleased with the results.