Carrick was one of only a few artists to use the goatskin jerkin in his depiction of the soldier. The painter Stanley Spencer portrayed soldiers in Goatskins in his Sandham Memorial Chapel Mural which enhanced his naive, homely imagery. The famous portrait and war artist Sir William Orpen also featured a goatskin jerkin in a number of wartime self-portraits including the ironically titles 'Ready to Start' self-portrait (see link above) and this is perhaps significant. As Robert Upstone mentions in the excellent critical edition of Orpen's war memoirs 'An Onlooker in France' some of the artists war paintings are reminiscent of Goya's war art and I think perhaps Orpen found in the goatskin a kind of romantic throwback to Goya's Geurilla fighters. In sketches such as 'Men Resting, the Bapaume Road, 1917' he continued to depict British soldiers wearing 'stinkers' (the name British Tommies affectionately gave to the goatskin due to the stench it gave off when wet) long after they had been replaced by the army issue leather jerkin.
Alternatively for Carrick at Killin the jerkin would appear to fulfill a number of functions. Firstly it is historically correct, providing a contemporary image relevant to the time. This was an item of clothing worn by many soldiers in the trenches in winter and would be recognisable as part of the uniform of the trenches. Yet although of the military it is hardly militaristic, being worn by soldiers to provide warmth and protection against the cold. This would have created a softer and more intimate image for mourners and might have found resonance among the community, the material echoing the warmth and affection which they felt for their lost men. Perhaps the source of this intuitive understanding of the importance of the texture of materials and the effects that can be achieved originates in Carrick's childhood, growing up in a house with his mother busily working as a skilled dressmaker.
Also the goatskin simplified the lines of the sculpture, eliminating much detail and the need to carve much of the soldiers webbing while also adding mass. This and the rough texture of the fur lended itself to Carrick's aim to create a modern standing stone by creating expanses of roughly hewn surface on the soldier's torso.
Finally if indeed Carrick based the sculpture itself on Michelangelo's David then the jerkin would seem to help further underpin this concept. The biblical hero was a simple shepherd boy. The Killin soldier's goatskin jerkin and soft bonnet (instead of steel shrapnel helmet) allows for a similar viewing of the Killin soldier, creating the rustic image of a modern shepherd who was not a professional soldier or a warrior by choice and who's prime motivation was the simple impulse to protect that which he cares for.