I like the baking soda better because my burn was a darker brown vs the borax turning black.

It's unfair to run all 3 tests at the same speed/power/focus settings. Just because the wood base is the same., the treatments you applied has efffectively created 3 different materials.. Have you tried changing the focus on natural wood? It's amazing that you can get quite a dark engrave with some diode lasers but it does depends on the focal length of your beam..Your "conclusion" is really an obsevation from the limited testing you carried out....... and they are perfectly valid. I twould have been more useful to explain why the differences . Wood is an interesting material made up of two main components, each with its own chemistry and damage threshold. The main component , cellulose, is a soft material and easily damaged by heat. It is encloses in thousands of porosus tubes of lignin (imagine fish net tights) . With untreated wood you are effectively vapourising the cellulose and scorching the more heat resistant lignin tubes. Add baking soda and the metallic salt in the compound, better absorbs the light energy. It still vapourizes the cellulose through the "fish nets" but better light absorption creates more damager (scorching) to the lignin. Borax is a completely different process . The Borax coats the "fish nets" and when you heat it the cellulose again vaporises but this time the heat turns the borax back into a fish net exoskeleton of boron. a black shiny metalloid substance . That black exoskeleton is very fragile and only dusty if you brush it and damge it. As you have seen for yourself , Borax gives by far the best results BUT you can add strength to the fragile black exoskeleton by spraying the finished engraving with a clear acrylic varnish. For food applications such as chopping boards boron is not toxic, It is found in many fruits and vegetables. Equally baking soda (sodium bicarbonate) is not toxic but produces more dutrable engraving. Both require surface protection if intended for regualr food use.. In effect what you have demonstrated with your test is that for the same parameters . 3 different materials absorb that light energy with different efficiencies.