I must disagree here - reversibility should always apply regardless if the fragment is part of an accessioned collection or not . Some historic sites may have formally accessioned their buildings and structures - many do not. That has no relationship to the proper stewardship and conservation of historic and artistic materials.

Before recommending any adhesive there are basic questions that should be considered. First, why did the piece of plaster detach? Is there incipient water damage, efflourescing salts, cracks, or other agents of deterioration present? To Elmers glue a fragment back in place on top of unsound plaster would only compound the damage in the future. 

By the way, Elmers and other white glues are PVA (polyvinyl acetate) and have bad aging properties - so if you want to ensure that something will fall apart again in 10 - 20 years then use the short term quick fix adhesives that you can find at any nearby drug or office supply store.

The ugly head is often seen by those of us in conservation when encountering bad repairs done without any consideration of the "dreaded" theories of long-term stability and reversibility.

Cheers!
"Dreaded" Dave

David Harvey
Conservator
Los Angeles, California  USA

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