Instead of getting a nice shade of black the wood just vaporises and where it actually goes dark there is very little contrast.
For most woods I would simply increase the power levels a little bit and be done with it but on those where the wood already disappears at low power there is a solution at hand.
I did some experiments to find way to protect the wood while at the same time increase the contrast levels.
One of the best things to do is proper preparation.
This means the surface of the wood must be really smooth, either by means of a planer or by sanding and cleaning with vinegar between sandings.
When the surface looks and feels like it is polished you are done if it feels rough continue sanding and vinegar clean the surface to get the wood fibres stand up.
Engraving the wood now already gives much better results compared to the standard surface but there is more:
A solution of dissolved Sodium Silicate (silica gel, crystal cat litter) brushed onto the surface and placed to dry, whill drastically lift the contrast levels.
Even with the wood fully dry the crystals inside the wood store water which is released during the engraving.
The sodium silicate will burn to a dark, blast dust, same a tiny bit of the wood - but without the laser really doing serious damage to the wood.
To get the same cutting depth as before you would now need about 15-20% more power in the parts soaked with the sodium silicate.
Only downside of the procedure is the fact that the black dust is even more sensitive than the charred wood, so you touch it before sealing and you leave a mark.
As for engraving settings with soium silicate:
Where I usually would use between 8 and 11% of power @ 4000mm/min on untreated wood I use between 7% and 9% power @ 6000mm/min with the sodium silicate.
A weaker solution gives similar results as a strong and concentrated solution, the only real difference is how much the laser still affects the wood.
If you do a lot of photo engravings you might want to try it out one day on leftover pieces
