Michelin has put out a visual indicator for dry-rot on their casings. Something about their molecular makeup allows the casing rubber to dry-rot to the extent that the entire casing is not safe to recap due to the heavy cracking after some years of service. Other manufacturers like Goodyear have no need for an indicator, so my theory is that Goodyear casings have a more dry-rot resistant makeup than Michelin casings.
Good idea. Older casings tend to out before a recap if they pass retread inspection... Depending on what brand the casing is, my local retreader reject casings older than between 4-6 years. They want a quality retread coming through their plant, which helps the dealer to stand by his product.