This pattern also requires silk roving, approximately 6oz total.

From No Sheep For You:

These ultra warm mittens are a take-off on the traditional Maritime design. Originally, thrummed mitts were knitted from leftover bits of raw wool fleece and sturdy lanolin-rich yarns. While the cotton won’t be as waterproof as wool, the raw layers of unspun silk will keep your fingers and palms toasty warm.

Designer Amy Swenson picked a sticky cotton for maximum friction with the thrums. Silk thrums. Silk fiber is longer and thinner than the wool that’s usually used, so to compensate, just fold the fiber more times than you would with wool. This will help take advantage of silk’s longer fiber length and also help better secure the thrums.