Granite/Marble: Black Granite is another easy material to use for engraving. Due to the high contrast, it is easy to produce bright, detailed images with the proper power and speed settings. The most common problem customers face is a lack of resolution, where the image seems washed out, or very grainy. This is caused by an improper dots per inch density or “DPI”.
The laser does not evaporate granite minerals, but heats it locally, creating a micro “explosion”, and the crater diameter is much bigger than laser dot diameter. SO DO NOT TRY HIGHER THAN 150 dpi, unless you know what you are doing. With a low power setting and slow speed and 1.5” focus lens, you may reach 200 dpi.
To reach high resolution, always try to enlarge the engraved photo size. At 150 dpi an 8”x8” (1200x1200 pixels) portrait will look 4 times better than a 4”x4” (600x600 pixels) portrait, because human eyes see total pixels, not dpi.
When you engrave a large granite monument, make your photo as large as possible to fill the whole stone surface, engrave at 125 dpi at higher power, to have bigger and deeper dots on granite, will result a sticking high resolution, high contrast and strong impact.
As a comparison, large banners are printed at 100 dpi, some highway bill boards are only 10 to 15 dpi.
Typically for black granite, the optimal DPI will be somewhere between 120-200, and can be tailored to fit your preference.
For off black darker granite (such as brown, red, sometimes dark pink granite) with obvious grain patterns, always enlarge the engraving size reduce dpi, increase power to get deeper and bigger dots, then highlight with bright white.
Marble is very similar to granite, but with finer grains. For black marble you can reach 200 dpi. However, for light colored marble there is no contrast between the surface of marble and the engraved area. Due to this, most customers use a color filler or paint to fill in engraved marble. This gives you a lot of freedom in your choice of color with your marble designs, and makes it easy to develop unique products.