During a good bit of the victorian period, especially the later part, it was steel, too...the whalebone had the consistancy more of some of the nylon boning and might not be firm enough for the later victorian corsets.
A properly made 18th cent set of stays would hold its shape without being on a form, it had so much boning.
After reading on real corsets, it makes the ones I have from Fredericks a joke. There's nothing that matches a handmade corset, especially a period piece. I also love traditional bloomers and garters too and ladies sleeping hats.