I've finally made a start on my planned 18th century wardrobe, and I've made that start by cheating, but only a little. As I want to make both a court gown and a polonaise, I've deicded to make just one set of stays for both dresses, and as much as I can, just one set of underthings for both outfits. The pattern I've choosen is the set of 1880's stays in Corsets and Crinolines. I've previously sized up and used the 1870's pattern, so I'm use the same technique to create this pattern.
The very first thing you'll need if your reference measurement, that is, a measurement on your body that relates to a place on the pattern that is going to be the basis for the rest of your calculations. Yes, there's maths involved in this, I apologise. For my refence measurement, I measured from the top of my lap to the line on my bust where the stays woul sit; for me this was 30cm, and this corresponded to the book printed centre front measurement of 7.8cm.
Though I took the measurement from my front, my own back being harder to measure by myself, I used the centre back patter piece to demonstate the principle of this form of pattern enlargement.
Having copied the pattern from the book onto parchment, I chose the bottom left hand corner of the pattern piece as my focus point.
I measured the centre back line as 9.8cm, and now simple cross multiplication with my starting reference measurements tells me that this needs to become 37.6cm. The rest of the sizing follows suit - pick a point on the pattern and draw a line through it and the focus point. Use cross multiplication to find out how large it needs to become, mark it off, and continue to the next point.
Thankfully, the back is rather simple, so I needed relatively few points to obtain the shape. The front pattern became a tangle of refernence lines very quickly, which was the reason I didn't use it for demonstration purposes.
If you use a pattern piece that, inconsiderately, doesn't have a nice simple line to the bottom, line this side piece here, just select the next best thing, for me, this was the lower most point on the pattern, as this still allows reference lines to be drawn neatly through all of the points on the pattern.
And is this the finished pattern? Well no, there's still work to be done. Yes, I've adjusted it so top to bottom, this pattern will fit me. But over the centuries, on average, women have been getting taller and broader, and given I am more voluptious again that the average, I'll have to broaden these pieces significantly to obtain the ideal shaping I'll need for the start of my 18th century silhouette.