I've moved into a Victorian house with these lovely intricate ceiling roses in a couple of rooms, but one thing that has bugged I have no idea how they are made? I can't see them carving them for every house surely, but they are far too complex to even multiple interlocking solid moulds. I could see how you could do it with lost wax and sand but then you have to somehow make the wax every time. These days you'd just use a flexible silicone mould, but obviously that wouldn't be an option. So what am I missing Reddit?
Composition ornaments are still being made and installed. They were originally a way to imitate much more expensive carved wood, stone or plaster.
The recipes for "compo" vary, but the basic stuff is whiting (chalk) bound with linseed oil, pine rosin and hide glue, worked up to be a firm dough. Individual elements are made by pressing the dough into molds- (your ceiling rose could have a lot of parts to it)- which then are pulled out when firm. Cornice moldings are also often given some burlap tags for more secure attachment to the wall or ceiling. The individual pieces are then softened with steam, which also makes the hide glue sticky, and assembled into place ( sometimes tacked through the burlap as well). Little bits of fresh compo are then troweled into the gaps and joints, and smoothed over to hide them. Then, of course, it's all painted.
The JM Weaver Company is one current maker, and has some more detailed info on their website about the process: