Is there a general consensus among Historians and Archeologists about how old a grave must be before it can be considered appropriate to open it to conduct historical research?

First off; I am writing this from my knowledge and experience in the UK (England) working with qualified archaeologists. As this questions ventures in to legal territory it will be different in each country.

In terms of general history, everything around 20 years ago is seen as ‘old’ enough to be historic, see the rules of this sub for more info. Generally once we go over 100 years, things become more interesting as you usually move out of living memory. In terms of graves and human remains, archaeologists and historians opinion is secondary to what the law says. It is also worth pointing out that most archaeology is done out of necessity by private firms funded by developers because of the legal requirements placed on development. For example if a road is going to be built an archaeological survey must be done otherwise the opportunity to dig there will be lost forever. Therefore most human remains are now found in building developments, and a lot of the time a desktop survey will identify the possibility of them being there.

In the UK the law states that all remains, no matter how old, should be treated with dignity and respect and you cannot remove a body from the ground on purpose (exhume) without a licence from either the ministry of justice, or if the remains are under 100 years old from the Human Tissue Authority which monitors how ‘recent’ human remains are being used. If the remains are on consecrated ground then permission also needs to be sought from the church. On non consecrated ground, from the land owner. I will get back to what happens when you accidentally find human remains later.

Originally in UK law you could not build on any land that was currently or had been used as a site for the internment of the dead, attached to a church or not. This law was relaxed in 1981 and you could now build on disused burial land, as long as notices were published and relatives of anyone buried there in the last 50 years were informed. Any objection and the development would be rejected. If the building is going to disturb human remains, then they must be exhumed and reburied appropriately, long term retaining of remains in museums etc. for research is considered by the Ministry of Justice on a case by case basis.

In practise, when a known (or suspected) grave is disturbed, the archaeologist will record, photograph, draw, map everything found. Including how deep, how it was positioned, its appearance etc. Items and remains will then be removed, and if deemed appropriate sent for further research or reburied elsewhere. In terms of further research, this will only be an object or a body that would substantially change our current understanding of a burial practice or if it is significantly out of place in that landscape. Mostly the thorough recording of the burial would be enough for any research.

So what happens if human remains are found where we really don’t expect? In UK law, if remains are found in a proper interment (a grave or burial site) or they are not over 100 years old, you call the police. Simple. Otherwise, they are subject to the same recording outlined above and often re-buried. Why 100 years? There probably isn’t going to be missing person or a current crime. So you could say the real answer to your question is that we only look at remains over 100 years old is because that is when the historian and the archaeologist, not the police force, become the investigator.