Why does adding liquid dishwashing detergent to mortar as it is being mixed keep the mortar from becoming stiff and needing retempering? It also makes the mortar smoother and more workable.
Liquid detergent acts as an air-entraining admixture. During mixing, the detergent forms tiny air bubbles that are distributed throughout the mortar. These air bubbles lubricate the mortar, making it workable for long periods, even as the water evaporates. Henry Plumber's book, Brick and Tile Engineering, cites a study that showed that increasing air content from 4% to 14% increased water retention from 70% to 97%. Water retention is a measure of the mortar's ability to maintain its workability, even after some mix water has been removed with a vacuum. I do not recommend using such detergents. Their uncontrolled use in mortar can greatly reduce the mortar's ability to bond to brick. It can also reduce tensile and compressive strength.