Hi Alan
I think Ben stated that
his resource said to coat the backs of the marble itself with Laticrete's 9235. Nothing is mentioned in that thread about adhesives and substrates.
If this were a industry wide issue, I think the quarry's would start implementing a coating to the back of the problematic tiles.
I currently have a customer whose White Thassos marble mosaic shower pan is discoloring. As I did my inspection, I observed the thinset was wet, but the mud pan was dry. Conclusion was the damp thinset was inhibiting a reaction with the oxides in the stone. An identical pan was installed as specified by the same designer on another project with no reported problems of discoloration. The second pan was installed with a surface waterproofing system. I don't know what thinsets were used on each project.
I do believe that several steps should be taken, and none of them involve using inferior products, and none of them will be a guarantee that discoloration won't occur. Too many countries and quarries in the marketplace these days. Adding a disclaimer is the best option, or actually recommend a change in material, or turn down the job.
If you think you can't afford those options, consider the alternative.
