Dip and drip, swipe or squeeze? The best method of draining water for optimal strength in a plaster of Paris backslab.

Plaster of Paris backslabs are used post-operatively to provide stability and protect repaired structures. We hypothesized that forceful expulsion of excess water during backslab construction could weaken the backslab by reducing the final gypsum content. Our aim was to compare the final dry mass and strength of backslabs prepared by three different techniques: 'dip and drip', 'swipe' and 'squeeze'. We applied an increasing force until the point of failure of the 30 backslabs prepared by the three methods. Backslabs prepared by swiping or squeezing away excess water were 9% lighter and 26% weaker and 13% lighter and 33% weaker, respectively, in comparison with simple drip drainage, and all results were statistically significant. We conclude that forceful drainage of excess water produces significantly weaker backslabs.