I always grey scale, by selecting “Threshold” not dithering for slate. Not too much power as it tends towards yellow, But it always looks good at the end. I spray the slate with a single coat of Polyurethane first as this gives the final engraving more contrast.
Here are some tips to implement before you fire your laser in order to get better results .
On slate prep:
Wipe the slate down with mineral oil first – it doesn’t require much oil, go light, and use the most lint-free rag you can wrangle. If you’re desperate, paper towels will work, but paper bits get stuck all over the place, let the oil soak in for about 5 minutes, then wash the board thoroughly with dish soap and water. This takes most of the oily sheen off the surface – when it dries, it’s still dark but not slick or shiny.
On file prep: there is very little play in greyscale here. Slate engraves to white almost immediately, so subtle shading is almost impossible. Crank your contrast way up, you really want almost pure black and white if you have any fine details.
sharpen the image, then desaturate to greyscale, then change the curve to make it almost pure black and white (curve should look more like a cliff in a graphics editing software if you have one.)
Power and speed should be based on your test pattern.
You set your speed to 6000 mm/min, but your firmware settings limit you to 500 mm/min. You also have laser mode disabled ($32=0), which will make the laser pause with every power output change. If you enable that ($32=1) you will need to reduce the requested speed. If the laser can’t move as fast as you ask, it will lower the output power to compensate.