Why shouldn’t you wear polyester dresses in a history setting?
- Polyester and other synthetic fibers melt when in contact with, or in close proximity to, fire. If your dress or under cloths catch fire, they can melt and stick to your skin.
- Polyester does not allow the body to breathe like wool or cotton will. If your body can not breathe during hot days or while doing period activities, you can overheat, get heat exhaustion, or even heat stroke. (In the case of a polyester covered bonnet, the heat can become trapped inside.)
- The vast majority of polyester fabrics will not work up the way wool or silk or cotton will. They require modern sewing techniques that defeat the purpose of period sewing for most.
- Oddly enough, mice like polyester. I don’t know why. Apparently, they think it is a yummy snack.
The cost: Yes, silk can be expensive. Yes, wool can be expensive. But, both can be had for less with patience and flexibility. I do not pay full price for my personal silks and wools. I can’t.
I took a quick look at what poly satin costs at the popular fabric/craft store. $9.99 to $14.99 a yard. Sure, that is less than $30/silk. But…. there are many other options out there. Here is a quick comparison of a couple. Currently, The Button Baron is offering limited runs of Family Heirloom Weavers fabric for $10/yard. Honestly, I would prize this type of dress over most nice silk dresses. Over on the discount fabric outlet sites, silk can be had for $10/yard or even as low as $6/yard. In each of these cases, the fabric is 55 to 60 inches wide, meaning a dress, even an 1850s dress, could be made with 5 yards. If you join some of the fabric resell groups, you can pick up dress lengths of silk and wool for $35 to $100 for longer lengths.
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If I Had My Own Blue Box: 
I won’t even buy a wool “blend” unless the “blend” is cotton or silk. If they didn’t have it back then, I won’t wear it.
Which a) fits your goals and b) is safe in a historic setting.