pictures: the whole stomacher, detail of tip
This one is meant to go with the 1680s manteau. Therefore, the two layers of foundation fabric are covered in the same fabric as the manteau's underskirt and the corset.
Sandwiched between the two layers of courtain fabic are bones of plastic that slightly fan out at the top. However, the stomacher would fold just like a fan if there wasn't any horizontal support, so there are three narrow bones inserted horizontally - one at the top, one near the tip, and one in the middle.
What makes the stomacher so special as to deserve a page of its own is that the top fabric was embroidered before it was mounted on the foundation. As the manteau is closed over the breast, only the tip of the stomacher would show, so this was a fine opportunity to try my hand at embroidery after something like 20 years' abstinence: If I messed up the first couple of leaves at the top, nobody would see.
The fabric - fine silk taffeta - required fine and shiny thread, and the fine thread required much work. I used Madeira Rayon thread, which is primarily intended for machine embroidery, so it's not much thicker than ordinary sewing thread. After the first two leaves I decided that using a single thread was too inefficient, and started to use two threads at once. That was a lot quicker and looked as good. Three leaves were done in vintage gold couching thread, and the spine of another in vintage silver thread. (All visible in the detail picture; the silver spine is in the lower left corner.)
Having finished the embroidery, I placed the top fabric onto the foundation, folded the allowance round to the back and neatened it up while sewing it to the foundation there. Then I attached four loops of fabric into which the needles will be stuck that hold the stomacher in place.
For my next embroidery project that involves fine thread, I'll definitely look at silk sewing thread or even special (and expensive) embroidery floss. Madeira, like buttonhole silk, is deplorably underendowed in the colour department: Baroque embroidery makes much use of multiple shades of one colour, but Madeira only offers two decent shades of green - at opposite ends of the brightness scale. Similar goes for blue and red. Don't even ask about yellow! Not to mention the fact that this stuff is, of course, not authentic, being synthetic.