Have you got it yet Wez?
I'd kind of agree with HTH on the scratchy-ness of brass slides.
I've got two:
The old brass-pipe I've had since 1982/3. I used to use this regularly in gigs back years ago and that would keep the playing area polished. I'd also give it a go with brasso to keep it nice...
But over the years I stopped bothering. If I haven't used it for a while it feels a bit dull, but soon "gets going" after half an hour or so.
The newer one is a Dunlop that I used to try out the resonator I bought at Vintage & Rare a few years back. I liked the slide so much that she chucked it in the case as a freebie :D.
It actually relegated my trusty old piece of pipe to second-best for a while. And then I noticed that the Dunlop actually had some sort of lacquer on it (to make it look cute, I guess) which wears off!!
Now, the Dunlop still feels "cleaner" inside but it plays pretty much like "ole trusty", so I use "ole trusty" instead for the mojo that's in it :lol:
It really is about finding something or somethings that do it for you.
For electrics, standard tuning, I tend to use a light clear glass slide for most stuff (think Rory, Duane, Ry, etc, etc). But a light chromed steel does a nice job too. It seems I want a light slide with as smooth a surface as possible on electric. The lightness cuts down on fret-clunking noises, and the smoothness cuts down the rasping on the strings. (However, if I'm playing open tuning with a raised action, ole trusty gives us the required rasp :lol:).
Ordinary acoustics, with lowish actions, I've never managed to master slide on. Probably the same clear glass would do a job, but there might not be enough volume/sustain.
On resonators with nice raised actions and 015s on it's a choice for me. If the strings are new, ole trusty is way too bright and I use a green bottle neck from Diamond (I believe). That is a righteous tone. However, when the strings are more than a couple of weeks old (and they stay on resos for a long time with me), the green bottle doesn't quite cut it. That's when ole trusty comes into his own. They pretty much do the same job tonally, as long as the strings under them are the right match...
It's all about getting a weight/density you're happy with and a size/shape that feels comfortable. Heavier, by theory, means more volume, it also enables you to get a consistent contact without having to apply too much pressure. However, the weight itself brings problems with physically controlling it. In the heat and excitement of a gig, I've found it's quite easy to groove your frets if you're using too bulky or heavy a slide... great slide tone but the guitar needs a fret-dress afterwards :lol:
(Amongst all this waffle, bear in mind that the finger you use is a factor. I almost exclusively put the slide on the weaker little finger. That's the way I learnt from watching Rory live (and an old bloozer I used to watch regularly in Exeter). I know I can get better control and tone out of my ring finger, but most of the time I want the first three fingers for fretting as well.)
Nowadays I believe that full appreciation of various materials and surfaces really comes later, after you've learnt to control this "over-size ring" on your finger.
Like picks, grab a couple of different (cheap) ones, use them as you feel like, but concentrate on getting the tones out of the instrument with your technique - it happens almost by magic if you keep fiddling around. Once you start getting your slide "voice", you can actually get it with whatever slide comes to hand, but you'll find different ones are more "suitable" for different songs or applications.
I'm personally coming from the other direction - I keep meaning to get a Steel like you'd use on a laptsteel, to try out on the resos. That might force me into wanting an electric lapsteel further down the line, but I just haven't got round to it...