Being the occasionally interesting ramblings of a major-league technophile.
Please note that while I am an engineer (BSCE) and do my research, I am not a professional in this field. Do not take anything here as gospel; check the facts I give. And if you find a mistake, please let me know about it.
Want something to last? Longer than ancient Egyptian monuments? Longer than cave paintings? Longer than the DNA of insects in amber? Depending on what, precisely, is being preserved, that may be possible... or not. If the preserved item is a living creature - with the idea of returning it to life - the longest of these example spans may be barely reachable. If you're intending to preserve a sample of human genetic material, that interval is hardly a tick on the clock.
Some of you have probably heard of scientists finding viable bacteria in a 250 milion year old salt crystal. Bacteria are pretty complicated, chemically and physically, so this is an astounding discovery, if it doesn't turn out to be a case of modern contamination. However, bacteria are physically small, and this makes them difficult targets for some of the agents which work against preservation. Ionizing radiation, for example. A human has a vastly larger cross section than a bacterium when it comes to intercepting cosmic rays or other background radiation. A single hit at the right place on a strand of DNA can turn a cell cancerous, and humans have lots of cells. Of course, if the human (or other large animal) is biologically inanimate at the time (as the bacteria above were for so long) this isn't as much of a problem, because the now-cancerous cell won't grow during the hibernation period. However, radiation does other damage besides cause cancer. A human preserved by a process intended to allow revival and put away for 250 million years would need some way of dealing with the damage caused by radiation during that time. And there would be other problems.
While, chemically, humans and bacteria are very similar, there are major organizational differences. Humans rely on complicated, large-scale systems in their bodies remaining physically intact for life to continue... or resume. Moreover, identity requires that certain neural pathways in the brain remain at least partially intact. Of course, those same systems also have extensive self-repair capacity. For that to work, though, the organism must be actively alive, not just floating inert in liquid helium. Long-term storage without some way to repair accumulated damage before revivification would probably fail.
Gross physical preservation is much easier than keeping intact all the multiple levels of complexity necessary for life. The main threat here is bacterial decay, and some methods of preservation are good at preventing that. Embalming and various methods of mummification are examples. Of course, if a body is outside weathering becomes a factor, as seen in some of the Andean mummies buried on mountain peaks in shallow graves. (One notable example's primary damage had come from lightning strikes.) If parts of their bodies are uncovered and exposed to the elements they are quickly stripped to the bone and hair. Even inside a structure, unless there are elaborate - and prone to failure - air conditioning systems, changes in temperature and humidity will cause problems. However, putting an object deep enough underground or in a sufficiently monolithic structure will mostly protect from those changes. As long as the cave or tunnel used doesn't flood or the pyramid collapse from an earthquake, and both remain unopened.
People talk about how well-preserved ancient Egyptian artifacts are. What they don't realize is that generally only the best-protected items are put on display, and even those may have been heavily restored. Even so, the degree of preservation is often astounding. This is largely due to artifacts and wall paintings being sealed in underground chambers, away from fresh air, sunlight and changes in humidity and temperature. Unfortunately, many of the tombs in the Valley of the Kings are prone to flooding during the periodic heavy rains which occur in the area. (In this case, "periodic" meaning brief, heavy rains every few decades.)
The builders of these tombs understood this problem, and built dikes to divert rain water away from tomb entrances. Even then, some tombs were damaged by water dripping down through cracks in the overlying rock, or when the dikes failed or were removed by later humans ignorant of their function. In one tomb, a guilded wooden panel was stripped of its gold by tomb robbers and left lying in the access passage. After 3 millennia of being alternately soaked in brief floods and then dried thoroughly by the intervening decades of desert air, it had - upon modern discovery - the structural integrity of cigar ash.
So, I've already mentioned radiation, light and water as enemies of preservation, as well as weather in general. What else do we need to consider?
Air. Mostly, its oxygen content, though other components may also cause damage. The paintings in tombs and caves have remained almost unchanged for so long largely because many of the pigments usd were made from oxidation products. Because oxides are already combined with oxygen - in fact, many of the pigments used owe their colors to this chemical alteration - they remain stable in appearance and physical characteristics indefinitely, even in the presence of air and damp. Iron is shiny grey and oxygen is transparent but iron oxide is red, for example. The other pigments were all either also oxides, or something else which did not readily react with oxygen. (Charcoal - largely pure carbon - is commonly used for black. Oxidized carbon is gaseous, but fortunately high temperatures are generally needed for the reaction to occur.) Some cave paintings are actually covered with a thin (so thin in some cases as to be transparent) layer of limestone deposit, due to the natural processes which build cave formations. This requires limestone-saturated water to flow over objects and evaporate, leaving the limestone behind. And some of the drawings have survived this!
Remember how the flowers found in King Tutankhamen's sealed sarcophagus still had a noticeable scent? After the sarcophagus was sealed the small amount of chemically active substances (primarily oxygen) remaining were quickly reacted, leaving nothing to attack the remaining scent molecules. Once the sarcophagus was opened these quickly oxidized and/or dispersed, and the remarkable scents were soon gone. The plants still retained a bit of color, as well, but the traces of chlorophyll and other pigments quickly oxidized once the flowers were removed from their static environment. The bleaching effect of the bright desert sun didn't help, either.
As implied above, oxidation can be used to "set" a molecular structure for preservation. Cross-linking between molecules of a substance can also do this. Sometimes this works so well it can cause problems later. During his examination of the contents of Tutankhamen's tomb, Howard Carter had just such a problem with the gold coffin and the inner sarcophagus containing it. After the coffin was placed inside its container a mixture of tree sap, pitch and mineral salts was melted together and poured over it. Most of this settled into the bottom of the inner sarcophagus, partially filling the gap between it and the golden coffin.
Gradually, through a combination of oxidation and spontaneous cross linking, the material hardened and polymerized into something which - by the time Carter started to work - even acetone wouldn't touch. Carter wound up suspending the inner sarcophagus inverted on sawhorses over spirit lamps, using wet cloths to protect the gold coffin. Slowly, as the mix softened from the heat over a period of several days, the inner sarcophagus was raised, until the coffin - settling through its own massive weight - was completely free, supported on the sawhorses.
Water primarily causes problems through three methods: Physical alteration of the substance through absorption and evaporation (as happened with the gilded panel above) chemical interraction and through transport of other substances. (Ground water in bedrock transports salt to surfaces, where the water evaporates, leaving salt behind to form slowly growing crystals, which expand in the rock or behind plaster coatings. As mentioned above it can also carry other minerals, such as limestone. It can even remove components from objects when passing through them.) Water can also act as a medium of life, turning a painted wooden door into a smogasborg.
The greatest short-term threat to preservation is biological activity. Bacteria and other life forms, after all, evolved to extract nutrition from materials by breaking them down, physically and chemically. Imagine a goat chewing on the Mona Lisa, or the paintings of the Sistine Chapel afflicted with a bad case of mildew. This decay is a problem more with organic materials than inorganic, and the closer the organic material is to the state it held during life the more vulnerable it is.
Mummies occasionally get mold or mildew if not kept dry enough. Most are very well preserved, through a combination of removal of the most vulnerable tissues (some of which are stored separately in canopic jars, where they are more thoroughly treated as well as being sealed away from air), desiccation and treatment with substances known to absorb moisture and inhibit growth of bacteria, mold and fungus. Of course, when a mummy does come down with something, modern caretakers can't simply repeat the original treatment; that would cause physical changes in the mummy, probably damaging it, and certainly affecting the provenance. A couple of decades ago I read about an ancient Egyptian mummy found to have a case of fungus, which was treated by exposure to ionizing radiation. This destroyed the mold while producing only minor chemical changes in the occasional molecule of the mummy.
Biological activity is only a problem where material exists for it to feed on in a life-friendly environment, and where organisms have access. Remove any one of those requirements and there will be no bacterial growth. One of the more famous demonstrations in the history of biology involved heating sealed flasks of nutrient mix to sterilize them. One flask was opened, the other left sealed. Naturally, in a few days the change in the opened one was obvious, while the sealed one was unchanged. This eventually led to a practical application in food preservation.
But even canned food breaks down, slowly. Some of the changes are chemical, some purely physical. The physical changes are due largely to settling of materials by density or size. (If you want to keep canned food for more than a few months, turn the cans over about every 6 months. This does nothing to maintain nutritional quality, but it does improve palatability by reducing the settling and separation which would otherwise occur.) So some method of preservation superior to canning is needed for storage of food for more than a year or two.
Drying has been used for thousands of years. Removing moisture from food not only removes a physical and chemical transport mechanism, it removes an ingredient important to life. If kept in a cool, dry and dark place, dehydrated food will retain a significant portion of its nutrient value for several years. (There's a strong resemblance between a salt-cured country ham and an ancient Egyptian mummy.) If sealed away from air (more specifically, oxygen - many companies provide dehydrated foods in nitrogen-filled cans) it will last indefinitely. Now, "indefinitely" is kind of vague. The shelf life of canned, dehydrated (either air dried or freeze-dried), nitrogen-packed food is generally given as 10 years, but truthfully we haven't been using this method long enough to say just what the shelf life really is. Most likely, food stored in this way, kept in a cool place, would retain at least a good portion of its nutrient value for as long as the can remained intact.
Another way to slow chemical activity is by lowering the temperature. The problem here is that some materials go through a significant phase change as temperature is lowered, water being the most obvious example. Not only does water freeze to sharp-edged ice crystals, but these expand to a larger volume than that occupied by the original water. Also, over time, the crystals - as crystals do - consolidate, moving water molecules from one place to another, turning a myriad of tiny crystals to many small ones. So you eventually wind up with a dehydrated material having a few large pieces of ice dispersed through it. There would be, of course, major changes in the texture and perhaps of the chemistry of the preserved material during this process.
The lower the temperature the slower this migration. At cryogenic temperatures it is pretty much - though not completely - stopped. Still, material stored in liquid helium would probably keep a long time before this process caused a significant amount of damage. The problem is that - on Earth, at least - maintaining such low temperatures requires a highly technical infrastructure. If you really want to preserve something a long time by freezing, you'd have to leave the planet. (The ice of the Antarctic ice caps is only a few million years old at most. Even without warming periods melting them, the pressure of new snow on top slowly squeezes the lower ice, causing physical deformation. Much of the ice is eventually pushed out onto the ocean by slow creeping due to the weight of ice above it and further inland.)
For really long term storage you definitely want cryogenic temperatures, whatever else you may do to preserve your material. Burying something in an icy planetesimal well out in the Oort Belt or Kuiper cloud would do for at least a few hundred million years. The temperature would be a few Kelvins, plenty cold enough. But there's one small problem. When other stars pass by, every few million years, their gravity affects the orbits of these bodies, sometimes sending one or more careering through the inner solar system. The chance of any one particular body having this happen is remote, but still... Even using a free body, one which orbits the galaxy independently of a parent star, runs this risk. And there's also a chance of collision, as well as cosmic rays. If you want to keep something frozen for billions of years this just won't do.
Interestingly, there's a place much nearer to Earth where a low-maintenance cryogenic facility capable of holding a very low temperature for a few billion years and providing better protection from cosmic radiation could be built. Find a crater at one of the Moon's poles. Dig a narrow, vertical shaft. Insulate the bottom and sides with multiple, waffled layers of something appropriate. (Aluminized or silverized mylar, for instance.) Place your object there. The deeper the shaft the lower the rate of micrometeoroid impacts and the lower the amount of outside radiation (including thermal) which makes it to the bottom. However, the Moon does generate some internal heat, so the hole can't be too deep. Half a kilometer or so should work fine. With the object insulated from the surrounding rock and open to empty space in a vacuum, temperatures will soon be down to not far above the microwave background. Devices of this type are known as Ruzic Cryostats. Of course, at the most, this will only last until the sun enters its red giant phase... No matter how hard you try, nothing lasts forever.
So, let's accept and value impermanence, and appreciate things for the few billion years they'll be around.
For information on the Long Now Project - intended to help humans think about long-term planning - check here: The Long Now Foundation.
This work is Copyright 2005 Rodford Edmiston Smith. Anyone wishing to reprint this material must have permission from the author, who can be contacted at: stickmaker@usa.net