Basically, I learned several lessons about dyeing fabric. Some of which are things I will definitely repeat every time I dye something; others are things I will never, ever do again. Let me share the learning with you.
The first thing I did was to read the instructions from the 1938 book by Mary Brooks Picken. Many times. It basically divided fabrics into two categories: cottons and silks/wools. It made a couple brief mentions of "artificial silk," which I took to be rayon. It emphasized over and over the need for the dye bath to get up to a high enough temperature or the dye wouldn't set into the fabric properly. It also said to boil the fabric. (Click here to read all of the original instructions.)
Boil? I was pretty sure you aren't supposed to wash rayon in hot water or dry it due to shrinking, and that's pretty much what all the advice I found online said, too. So I found the smallest bit of fabric that could be spared from the 30s ensemble: the fabric belt loops. I took one and boiled it for half an hour, measuring it before and after to see if it would shrink. As you can see, nothing much happened:
So I considered it a go to boil the rayon, although I still had some major reservations. I thought maybe the rayon had been treated, or that it had been pre-washed by the seamstress that made it or any other number of situations that would keep the belt loop from shrinking. I had left the other fabric pieces in the sink to soak in clean water, so they were wet and ready to be boiled. (The 1938 book said to put wet fabric into the dye, not dry, to prevent streaks and blotches.)
I put in 3 gallons of water and half a bottle of the Rit Pearl Gray dye, along with one cup of salt. I stirred it a LOT to make sure everything was thoroughly mixed before any fabric got to it. This prevents blotches and streaks, too.
When the water was lukewarm, I tossed the fabric pieces in and let them sit for 15 minutes at that temperature. Next, I raised the heat to boiling (well, steaming really, not a rolling boil), and left it in for 30 minutes. (I stirred the entire time. That is important. I also made sure to try to keep pieces untwisted and to give each piece equal time at the bottom where the heat is greatest.) I turned off the heat and let it sit for another 30 minutes, then rinsed it all in cool, clear water.
As I began hanging the pieces outside on the line to dry, I noticed a horrible thing. I think the fabric shrank. But mostly in width, not evenly overall. While some people recommend trying to re-stretch rayon that has been shrunk while it's wet, DO NOT DO IT. (I didn't, no worries.) Rayon, especially vintage rayon, loses a lot of its strength when it's wet, so the odds are good that you will tear the fabric rather than get it back to its original size. Resist the temptation!
So, currently, the shrunk fabric bits are hanging on the line and drying thoroughly. I think I will give the dress pieces one more dip in the dye bath to match it up with the jacket pieces. (The jacket had not faded as much as the dress had originally, so they differed in color a bit to begin with.)
After having a dramatic moment last night after realizing I shrank the fabric, I have come up with a plan to save the suit. I'm following some recommended ideas for adding width to garments found in the 1938 book, and I think I'm actually going to end up liking this piece more than before! However, until some rescue fabric arrives we're going to take a brief recess on this project.
Lesson learned, I stirred beautifully and heated too much. If you plan on dyeing rayon, listen to others. Do it in cool water. But do it with lots of stirring, 'cuz even though my pieces are now too small, they are beautifully, evenly dyed with no blotches! At least I got THAT part right! Live and learn. This is why you learn with vintage that isn't in good condition… In case you didn't know, bloggers aren't perfect and don't always do things right.
Update: In a Twitter conversation with Rit dye, they affirmed that the dye does need to have a high heat temperature to set into a fabric. However, garments can still be dyed successfully by using warm water (if both the garment and intended dye color are light).
If you want to read the original book instructions, click here. (They're pretty long, so I figured a pin would be better than on here!)