Stefany, I think they are cold decorated with fire-on enamel type glazes, or some other type of overglaze that is fired on in a second heating process. It could be brush applied, but to me looks like syringe application of a finely ground liquified glaze material. That's how it looks.

I might be able to research more. I encountered one article about makers of reproductions, but don't know if it is this particular manufacturer.

If it was trailed stringer glass as we see in Venetian beads, there would be thickness, even texture, of the applied decoration. This material flows slightly and creates a thin clean line. Interesting actually, I am not familiar with the process.

EDIT: Apparently they DO use glass stringer. Please see the next post.