How do strappo and stacco differ as methods for removing damaged frescoes from walls?

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Charles Sprague Pearce: Religion — Mural Detail of *Religion*, mural in lunette from the Family and Education series by Charles Sprague Pearce, 1897; in the Library of Congress, Thomas Jefferson Building, Washington, D.C.

The strappo and stacco methods are techniques used to remove frescoes from damaged walls, differing primarily in the amount of original plaster retained during the process.1

Strappo: This is a more radical procedure. A canvas is glued firmly to the fresco's surface, and the thin layer of plaster containing the pigment particles is pulled away. Because the bond between the canvas and the fresco must be stronger than the plaster's internal cohesion, the resulting layer is very thin. The back is then thinned further and fixed to a rigid support. This method is now seldom used because it can irreversibly alter the original surface character and pigment density.1
Stacco: This method is considered less intrusive than strappo. It involves removing the fresco along with a thicker layer of the original plaster. This layer is smoothed flat on its back before the painting is mounted onto a prepared support.1

While these methods are used for removal, the art conservation community generally prefers to perform treatments in situ to preserve the painting's architectural context and site-specific meaning.1