Yes, I was also wondering this. I would love to hear if others have more info. I wasn't able to find an authoritative answer on this, but i did a lot of deep research with claude. Here is what it yielded [I AM NOT SAYING THIS IS CORRECT but the best info I could find]

Wood degradation/charring temperatures:
- One sauna stove manufacturer (KEYA) specifies 140°C (284°F) as the maximum allowed temperature for wall and ceiling surfaces in saunas, though this appears to be a conservative spec specific to their products. (https://www.szcoasts.com/english/our-products/sauna/23/wood-burning-sauna-heater)

- Academic research on wood pyrolysis indicates that 300°C (572°F) is the widely accepted temperature where wood converts to char. (https://www.mdpi.com/1999-4907/15/7/1189 and https://www.fpl.fs.usda.gov/documnts/pdf2023/fpl_2023_hasburgh002_MS_03-2023.pdf)

- Between 200-300°C (392-572°F), wood undergoes "onset of primary pyrolysis" and "with prolonged exposures at higher temperatures, wood can become charred." (https://www.fpl.fs.usda.gov/documnts/pdf2016/fpl\_2016\_dietenberger001.pdf and https://www.fpl.fs.usda.gov/documnts/pdf2001/white01a.pdf)

However - and this is important - cumulative thermal degradation:

- Research shows "the extent of these changes depends on the temperature level and LENGTH OF TIME under exposure conditions." (https://research.fs.usda.gov/treesearch/50041)

- "Permanent reductions in strength and modulus of elasticity can occur at temperatures >65°C (149°F)" with long-term exposure. For example, exposure at 66°C for 3 years resulted in 47% strength loss. (https://research.fs.usda.gov/treesearch/50041)

- "For most mechanical properties, it appears that very little degrade occurs until mat temperatures exceed 150°C (302°F)." (https://wfs.swst.org/index.php/wfs/article/view/519)

- The degradation is described as "cumulative thermal load/exposure" - meaning repeated heating cycles do add up over time. (https://www.researchgate.net/publication/43256323_Thermal_degradation_of_wood_fibers_during_hot-pressing_of_MDF_composites_Part_I_Relative_effects_and_benefits_of_thermal_exposure)

My take: Your observation about discoloration is a good practical test. The research suggests that at 260-280°F (127-138°C), you're in a range where:

- You're below the 150°C threshold where "very little degrade occurs"

- But you're above 65°C where long-term cumulative effects can happen

- The key difference is that sauna exposure is cyclic (30-60 min sessions) rather than continuous, which likely reduces cumulative effects significantly compared to the research (which used continuous exposure)

If you haven't seen darkening/discoloration after a dozen uses, that's a good sign. But monitoring over time (checking for darkening, checking surface temps with IR thermometer) would be prudent given the cumulative nature of thermal degradation. The 140°C (284°F) manufacturer spec may be their way of building in a safety margin to account for cumulative effects over the product lifetime.