The paper summarises a series of tests carried out on gypsum-sand mixes—using different sands and mix proportions—and undertaken with a view to reliable information on base mortars for gypsum-sand plaster, in the absence of a local consensus and clear-cut instructions. (The current standard specification prescribes the mix proportion irrespective of the type and quality of the sand, while gypsum manufacturers recommend mix design based on a predetermined water-R.H.H. ratio, again irrespective of the sand quality). A uniform consistency was used throughout, based on the average accepted in local practice.