I often field questions about mixing yarn types. While it is true that different fibers will take-up or rebound differently and they may shrink differently during wet finishing, how much this will affect your final cloth depends on the proportion of each fiber in the cloth, the elasticity and fiber content of the yarn, and your choice of wet finishing.
My general advice is to be fearless. Give it a try and see what happens. There aren't as many constraints as you might think. Keep in mind, I'm writing from a rigid heddle weaver's point of view. The constraints and freedoms of this loom type may be different than other looms. (All looms have constraints and freedoms. I wrote about my thoughts on this subject in this post.)
If the yarns are used at the same or similar proportion in the overall fabric, then their differences tend to balance each other out. This doesn't mean the proportions have to be exactly the same, but rather that you keep the proportional relationship between the two consistent throughout the cloth.
For instance, these two sets of towels from the Weaver's PlayBox use three different yarns. They have been machine washed and dried three times. These yarns all share a cellulose base, but how they are made and the types of fiber used, are quite different.
Duet, shown in Chambray (blue), is a cotton linen yarn made by plying a strand of tow linen and two strands of slub cotton together; Mallo, shown in natural (white), is a softly spun slub yarn; and Brassard's 8/4, shown here in Raspberry (red), is a 4-ply un-mercerized cotton with moderate twist.
The Duet shrinks and takes up less than the Mallo, creating a nubby, textured fabric. Mallo and Brassard's differences can be seen more at the selvedges, creating a scalloped edge of the checked fabric. (Straight selvedges are not the end-all/be-all of fabric making.)
Sett and structure also plays a role. To keep the fabric smooth, I aim for a balanced or slightly open sett to give the yarns room to move and settle without interfering with one another. Close setts can lead to more puckering. (If you aren't familiar with the terms “open” and “close” in relation to sett, I discuss them in this blog post.)
In the example above, the yarns all had similar elasticity. What if your yarns have different elasticities? The same general rule applies if you use them at the same or similar proportion in the overall fabric then then their differences tend to balance each other out. It is possible that you may get some waves in your fabric. In most cases, I see these textural differences as a strength. If that bothers you, in most cases, a steam press will flatten the fabric.
Mixing wool and cellulose together is something that gives a lot of new weavers pause, but this kind of fabric has been woven for centuries, linsey-woolsy comes to mind. Both of these swatches were woven on a sett of 8 and using the singles yarn from the wool PlayBox and Duet, then machine washed. The proportion of wool to cellulose is different in each swatch and creates a different effect in the each cloth. Both are equality promising as a project fabric. Your results can vary based on the types of wool and cellulose fibers that make up your yarn.
If you wish to mix yarns of really different construction types, for instance a boucle, slub, and tape yarn, intersperse them with a smooth, well-plied yarn at least an end or two between each one. This will keep you from getting a sticky shed. You can also consider using an open sett so the yarns don't interfere with one another, but you want to be careful that the fabric will hold up to use and doesn't fall apart or snag. Mohair is often used to keep open setts stable.
When it is released from tension, take-up occurs. When washed, the yarns bloom and settle. Almost all mixed warps can be hand washed to preserve the look you are seeing once the fabric is off the loom. With a gentle hand wash, the yarn doesn't shrink as much as they do when machine washing.
In some cases a rougher finish will give you interesting results. Fulling wools is an ancient practice and it may feel counter intuitive to throw your wool yarns into the washing machine, but it can create some pretty terrific fabric. (In the Spring 2021 Weave-Along we played around with the art of fulling fabric or shrinking it on purpose.)
Below is a scarf fabric from the Weaver's PlayBox focusing on wool. On right is the fabric hand washed and on the left is the same fabric that has been machine washed on gentle. The fabric on the left has lost considerable amount of width, it is denser and as a result more waterproof, but the hand of the fabric is still subtle. This technique is often used when creating outerwear. A bonus is you can cut and shape the fulled fabric without the fabric falling apart.
Testing your ideas is one of the best ways to see if your particular mix is going to give you the results you want. The rigid heddle loom is a particularly sample-friendly tool since it is easy to warp up and creates minimal loom waste.
If you are interested in learning more about yarn, fiber types, and yarn construction, I wrote a little book just for weavers called, A Weaver's Guide to Yarn. You can get a digital copy right here at Gist.
Smitten by small looms and big plans, most of my weaving life is focused on creating know-how for the rigid-heddle weaver including collaborating with Gist to create the PlayBox series. I host Yarnworker, a site for rigid-heddle know-how and inspirations and its companion learning space Yarnworker School. From my home in central New Mexico, I weave, write about weaving, and dream-up, film, edit, and host the weave-alongs and classes. This space is supported by an amazing Patreon community who make all the weaves possible.