Will, regarding a mason's apron... I am a fan of Thomas Hardy, and I am now reading "Under the Greenwood Tree, or the Mellstock Quire". Thomas Hardy's father was a stone mason, and besides being a writer, Thomas Hardy himself played the fiddle. In his 1896 preface he wrote about the meaning of the book..."This story of the Mellstock Quire and its old established west-gallery musicians, with some supplementary descriptions of similar officials in "Two on a Tower", "A Few Crusted Characters", and other places, is intended to be a fairly true picture, at first hand, of the personages, ways, and customs which were common among such orchestral bodies in the villages of fifty or sixty years ago."
"One is inclined to regret the diplacement of these ecclesiastical bandsmen by an isolated organist (often at first a barrel organist) or harmonium player; and despite certain advantages in point of control and accomplishment which were, no doubt secured by installing the single artist, the change has tended to stultify the professed aims of the clergy, its direct result being to curtail and extinguish the interest of parishioners in church doings. Under the old plan, from half a dozen to ten full-grown players, in addition to the numerous more or less grown-up singers, were officially occupied with the Sunday routine, and concerned in trying their best to make an artistic outcome of the combined musical taste of the congregation. With a musical executive limited, as it mostly is limited now, to the parson's wife or daughter and the school-children, or to the school-teacher and the children, an important union of interests has disappeared."
In the first chapters he introduces the characters that make up the quire, or band of church musicians, and the members of the group assembled at the house where they have met to practice. The maternal Grandfather of the family is a stone mason, and he describes him..."Being by trade a mason, he wore a long linen apron reaching almost to his toes, corduroy breeches and gaiters, which together with his boots, graduated in tints with whitish-brown by constant friction against lime and stone."
And so Will, to answer your question, the long mason's apron protected the worker's clothes from mortar and lime and rough abrasions from the stones.
Alice in Montana