One of my hobbies is collecting ancient coins. A 2000 year old Roman sesterces may seem old in terms of a human life span but it is a flash in the pan compared to geologic timescales. I've handled sesterces (bronze) that if the patina was ignored would appear to be in mint condition. A sesterces can be made shiney by dipping it in acid and then buffing it but doing so reduces the coin's value. A sesterces that is in good shape almost always comes from the center of a coin hoard. Coins from the exterior of the hoard that came into actual contact with the soil are normally very corroded. Good quality silver and gold do not corrode. However silver has a funny property that it becomes as brittle as glass when it is over a thousand years old. People have dropped ancient silver coins and have them shatter into dozens of pieces. Lead is not all that stable a metal. The medieval Europeans had the tradition of burying the Holy Roman Emperor with a commemorative plaque and mock regalia made out of polished lead, e.g. sword, crown, scepter, etc. If you pop open the coffin of an old emperor, all you'll find is old bones, black carbon from the decayed flesh and lots of white powder from the lead regalia (very frustrating for the archeologists). In Lefkadia, Greece there is this fascinating Macedonian tomb that held the ashes of either Philip II (Alexander the Great's dad) or Philip III (Alexander the Great's congenital idiot half brother). Classical scholars love to argue about which Philip it actually was (nobody knows for sure). This tomb was filled with iron artifacts, e.g. swords, spears, lamp stands, etc. When the archeologists opened the tomb, they found all of the iron artifacts had turned into piles of red dust (rust). The gold larnax that held Philip's ashes from cremation was still in pretty good shape (like new actually). That's the neat thing about gold, it does not age or corrode. Anyway, take all that information and make it the context about what would happen to an alien heatshield left out in the open on Earth for 10,000 years (not intombed).

Now we can have some fun with science fiction. For example, imagine an alien probe landed near the ancient Egyptians. Assume the Egyptians venerate it like the Moslems venerate the meteor of the Kaba stone. Assume the artifiact is made of titanium (very corrosion resistant material) and the Egyptians bury the artifact in an underground tomb in the super dry climate of the Sahara desert. Such an artifiact could be in mint condition. That's the only scenario I can see where an alien artifact could survive on the Earth (very unlikely).

Gary