Cotton fabric is a great option for nuno felting year-round but especially when the weather gets warmer; it provides a durable canvas for everything from nuno felted shapes to garments and accessories such as purses and bags. (Cotton really isn't just for the warm months and warmer climates - it's such a wonderful fabric to use and wear.)
For most projects the lighter-weight the 100% cotton fabric the better it works. I've used Cotton Voile, Scrim, Hospital Gauze and Dharma Trading's Harem Cloth.
I dye all of these cotton fabrics myself even though many 100% lightweight cottons are available in commercially dyed colors and prints. It's harder to dye cotton whatever method you use so it's usually best to over-dye to compensate for the inevitable color-loss during the nuno felting process.
Dharma's Cotton Harem Cloth is my new favorite (even better than voile) because it can be over-dyed and has so much more body & crispness to it --- I love the way it holds up to the tough nuno felting process. (You need to cut it with scissors, not rip it.)
Cotton has characteristics that make it very different from working with silk - the 2 main considerations are WEIGHT and TEXTURE.
All the cottons including voile are heavier than most lightweight silks (3mm to 5mm) so the weight of the cotton needs to be factored in to your end product. If you are making wearables the weight affects the final drape. If you are making bags, hats and other accessories, cotton may be the better base fabric because of it's weight and durability.
Texture is a dominant feature when you use cotton as your base fabric because it does completely different things when it shrinks during felting and can be so dramatic that it's actually as much of a design-decision as it is a fabric-decision.
Cotton (whatever type or weight) wrinkles and puckers, folds and pleats and you can pump it up or tone it down depending on how much and where you place your fibers in your design layout.
For example if you spread out your fibers similar to the prep for Cobweb Felt you'll end up with tons of small puckers between the fibers - a fascinating and wonderful effect.
If you vary the direction of your fibers in the initial layout you'll get puckering in both directions and you can really play up this textural-effect trick with the heavier weights.
COVERING EDGES AND ENDS
All the cottons fray like crazy so it helps to think ahead about how to deal with it during layout. I like to use the Wrapped Edge Technique because it looks more professional (and hides all those ugly edges and ends). You can also serge the edges, use a Rolled Edge foot on your sewing machine or buy cotton scarf blanks with finished edges.
TIP: COTTONS THAT WORK FOR NUNO FELTING
gauze (from tight to loose weave often labeled "hospital gauze")
Dharma Trading's Cotton Harem Cloth
lightweight cotton blends with at least 80% cotton content