Updated January 21, 2022.
That picture is the daybed in my studio with many of the threads I’ve picked for my rug spread out on it. The rug uses motifs from Owen Jones’ Grammar of Ornament and so that is there as well. Obviously, I have lots of stash.
Having stash to use is great for rug because you don’t need to have colors match exactly unless it is the background. When a rug is broken up as this one is, even the background can change. Mix dye lots, threads, as long as you stay in one color palette, it’s all good.
The very best thread for rugs is silk and I will be using little bits of it. But silk is expensive and rugs are large. The second best thread for rugs is wool and the bulk of my rug will be in wool.
There are three main types of wool to consider. Persian Wool, originally developed to repair oriental rugs, tapestry wool developed for needlepoint, and crewel wool, developed for embroidery on cloth. All three can be mixed and used in a rug with no difficulties. You will find the biggest variety of manufacturers in crewel wool. You should also look at wool knitting yarns if you need to buy thread for larger areas. It is a significantly better buy. Just be sure to get the correct weight. In my pile are all three types of wool.
The third best choice is wool blends. These come in two types, all-natural blends, like the silk/wool blend such as impression, Essentials, or Silk & Ivory. You can also use one with synthetic threads, such as Rainbow Tweed and Burmilana. The synthetics may not wear as well as pure wool and the wool/silk blends may be too soft for a rug.
All blended threads run the risk of pilling. Before committing to using these for a large projet, stitch a small project to check for this.