Another shot at a Houskeeper seal. This time, I heated the bare copper red-hot and dipped it in a tub of glass powder. Some melts and sticks to the copper which on further heating forms a nice glass layer. First three photos below show the effect achieved.
I then tried a proper seal. The glassing went OK, but when I tried sealing on the glass tubing (test-tube material again), it appears not to have stuck at all to the initial glass! Maybe different kinds of glass?