This wing chair is in my client's garden room. The fabric is a cotton toile, but when I washed it before cutting and making the slipcover, it came out of the dryer and felt just like a very soft flannel. It seemed like a different fabric.
Most fabric can be washed, despite the 'Dry Clean Only' warning on almost every home decorating fabric. The manufacturers do this to cover themselves. Some you obviously wouldn't wash, like silk. But the only time you really can't wash a fabric is when it is an upholstery fabric backed with rubber. That won't wash, at least not without ruining the fabric. Other than that, I say, wash it!
It is kind of hard to see on the chair, but I also made large, ruffled pillows for a bench that is in the room, as well. I just forgot to take a picture of it. The wing chair was on the large size, so the pillow actually works with it. A lot of times, wing chairs are more compact and an additional throw pillow would make sitting in the chair hard. That was not the case with this one.
I covered the buttons with the solid part of the fabric, just so they would show a bit more. Didn't really help as it is such a busy, dark fabric.