About Guitar Slides:
Slides may be used on any guitar, but slides generally (and steels, in particular) are often used on instruments specifically made to be played in this manner, such as steel guitars, resonator guitars, lap slide guitars and electric guitars.
An ordinary guitar, either electric or acoustic, can be used for playing slide. Often the strings are raised a little higher off the neck than they would be for ordinary guitar playing. This is done especially if the free fingers are not going to be used for fretting.
Around 1975, glass guitar slides started appearing in music shops across the United States. Clear Glass Manufacturing Company and Clayton Products supplied the majority of the market. Both companies grew, though eventually Clear Glass was sold to Dunlop Manufacturing. Modern bottleneck slides are still manufactured by Dunlop, as well as companies such as Steve Clayton, Inc., Mr. B's Bottleneck Guitar Slides, Bluemoon Bottleneck Company and Diamond Bottlenecks.
A slide can be made with any type of smooth hard material that allows tones to resonate. The slide's weight causes differences in sustain, timbre and loudness, while the surface structure and material affect tonal clarity and timbre. Heavier bottlenecks usually can produce longer, warmer and louder tones, but they also require more mastery to play with.
With the sliding technique originating from the cut-off neck of a glass bottle, bottlenecks usually still have the same tubular shape and a length of one to three inches, and glass still remains a popular material. Glass coloring also alters the timbre. The addition of iron oxide turns glass green and hardens it, causing a louder and sharper tone with a warmer sustain, while cobalt oxide colors it blue and produces the sharpest, loudest tones.