I received a private PM that I thought I would share here.
"Hi Tim,
I was wondering why you added the 4 extra dark strips on the back of the ziricote guitar. Are they additional protection against cracking. I have heard that ziricote is more prone to cracking from humidity changes than other woods."
My response"
Yes, Zircote has a reputation of cracking. When we exhibited at Healdsburg in the late 90's or early 2000's there were 3 Zircote guitars which cracked DURING the show. Granted the conditions were brutally dry in that venue but I did witness it and it made quite a few of us take notice. The show promoters brought in some portable humidifiers but its was too little and too late.
Since then I have added very thin cross grain reinforcement strips (mostly to Zircote) and have yet to have any of those guitar backs crack. The black strips on this guitar are made from African Blackwood. The sides are doubled so there is no chance they will crack. Humidity doesn't seem to be as much of a problem as drying conditions. This guitar is going to be shipped to more humid conditions on the other side of the globe so that climate is better suited for Zircote than say Arizona or Africa. BTW, those additional strips don't negatively damp the back vibrations or affect the tone.
You might also notice that I am spraying sealer on the inside of the back, sides (and top). The sealer doesn't completely stop moisture exchange with surrounding conditions but it does drastically slow down that exchange. Its rapid changes that adversely effect wood movement but if wood is allowed to slowly acclimate to its surroundings then slower movement usually isn't as much of a concern and the wood is able to adapt to those slower changes more easily.
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