Normal circumstances that sets upon death:
But it works differently for as far as any living beings are concerned. From the onset of death, the process of decay almost starts immediately upon death. Then, maggots start to do what it is supposed to do to any dead being leaving just bones in the course of time. (Refer images below)
The bodies of Saints that defies normal process of decomposition.
Some prefer to call this phenomenon as incorruptible bodies but I beg to differ in the terminology used. This is because it is still corruptible as some form of decay does take place such as sunken eyes, darkened eyes, or hands that need to be touched up with wax. There are also some cases involving embalming which is far from the reality of the said incorruptibility.
St. Bernadette Soubirous (7th January 1844 - 16th April 1879)
In another words, the bodies of these saints continues to be breaking down under the clothes or robes they are wearing but at such a slow pace. In some of the cases, parts of the bodies have been removed in the case of St Bernadette below. A thin layer of face mask has also been constructed to cover her actual face because the nose and eyes have sunken in. Even the fifth and sixth right ribs were removed at the request of the Bishop to be kept as relics (Dr. Comte's report) in Rome. It has been observed that the bodies have taken a greyish shade with mildew patches, large amounts of crystals and calcium salts. A designer in fashion mannequins, Pierre Imans did some finishing touch for the hands and face (light wax mask) giving it a smooth and fine texture.
Normal human beings would only be left in bones within a year. Somehow, despite some delayed process of decomposition, it is indeed a miracle that bodies of Saints can stand the test of time. It is still not known what constitutes to this condition, whether it is the diet, the simplicity in thoughts or the meditation that has changed this normal biological process of decomposition known to mankind.
2. Bishop of Nevers
3. Dr. Comte