Choosing a Slide: Mass, Hardness, Materials, Fit
Slides are widely available in different designs, materials, and sizes. These comments will hopefully save you from buying slides that will not serve you well and save you from picking a musical tool that might discourage you from pursuing slide in your musical life.
Your choice of slide may very well change with your choice of guitar, string height and gauge, and amplification set up, as well as whether youíre recording or performing live, playing solo or in a band context.
That said, hereís a quick breakdown of the options and their ramifications.
The mass of the slide is fairly critical to sound production on acoustic guitars.
Electric guitarists can get away with thin slides made of a wider variety of materials because their amplifiers are doing much of the heavy lifting. A judicious use of volume and compression to boost the signal will allow them to play slide with almost anything.
Acoustic guitarists will best choose slides with significantly more mass. Up to some reasonable limit, thicker slides more successfully isolate the damping effect of your finger from the strings, absorb less of the mechanical energy of the plucked string (surface dynamics...see below or ask your friendly local physicist), giving you more sustain and volume.
Tone and control of ambient string noise will vary according to the hardness of the slide material. I have used slides made of copper, tempered pyrex, glazed porcelain, hand-blown glass, brass, a relatively soft stainless steel, and chromed pot-metal. The copper was far too soft for acoustic guitar: the sound dies almost immediately. The chromed pot-metal and stainless are too hard, the tone being dominated by a high end sizzle which emphasizes string noise while being unpleasantly thin.
Buying Advice: Which Material?
When choosing a slide for an acoustic guitar, the best choices tend to lie in the middle of this hardness range where sustain is balanced against string noise.
Heavy-walled glass, porcelain, and brass slides generally provide enough high end to give a crisp attack, enough sustain and volume to give you a good sound to work with, and are still soft enough mute a pleasant amount of the ambient string noise. With digital recording, and some contemporary amplification choices (which often tend to boost high frequencies significantly), your slide choice may be affected.
The choice of a wooden acoustic or a resonator guitar will also affect slide choices, due largely to the frequencies the different instruments emphasize. A little experimentation with the variables in your own set up is always in order. As with many things in life, there are lots of right answers, and only a few wrong ones.
String Height and Slide Set Up
While there are sensible principles that will influence your slide and set-up choices, there is no single, Ďlittle-cí catholic standard to which youíll need to conform. That said, higher action and heavier strings are generally preferred. This does not mean that you need a junker to play slide. Sadly, when you see a guitar in a music shop window that says ďGreat for SlideĒ prepare yourself for high action, a neck that might need a reset or a broken truss rod.
My strings are neither as low as a finger stylist might like, nor as high as a slide guitarist might like. We all cut our deals with these particular devils.
The slide should fit your finger snugly enough to keep from falling on your toe when you relax your hand at your side, fingers straight down. This will keep your hand muscles from having to hold the slide in place on your finger, freeing them to play guitar, which after all is the point. You can pad an ill-fitting slide with leather, rubber, or adhesive-backed foam like the stuff sold to winterize windows in your favorite hardware store. I have weathered many an existential crisis wandering those aisles. Good therapy.
To maintain the maximum flexibility and usefulness, I suggest learning to play with your slide on your little finger. This is where Robert Johnson had to have kept his, given the sorts of figures he played. This placement leaves three adjacent fingers free to play figures behind the slide and for chords leaving the slide out of the way.
That said, Iíve seen players, like my friend Mike Dowling play around the slide worn on his ring finger, and his style is developed around that. Bonnie Raitt wears her slide (a Mateus wine bottle neck) on her Ďredneck saluteí finger, which really does limit what she can do with her fingers, but whatís wrong with her playing: nothing!
As an instructor, if Iím starting you off (or if I catch you midstream), Iíll do my best to convince you to wear your slide on your little finger. Sometimes, I fail. I can deal with that.
Physics, Surface Dynamics and Slides
Recently, I appealed to John Venables, a physics professor at Arizona State University, for a scientific perspective on the acoustical qualities of different materials, and giving him the credit for whatís right about this (and taking the blame for any misunderstandings), this is what I gathered:
- Different materials have different rates of energy absorption depending on their hardness, stiffness, and ability to convert sound energy into heat (vibrations typically at infrared frequencies that warm up the molecules by wobbling them against each other). The more readily a material absorbs the mechanical energy of the vibrating string (converting sound energy into heat), the poorer the materialís acoustical properties.
- The ability to convert vibrations into heat by absorbing the energy involved, often depends not on the crystalline solids themselves, but on the defects (known as Ďdislocationsí) within these solids. The properties of these dislocations can be changed by tempering, also known as annealing.
- A starting point might be to think about what makes a good bell. These are cast and then tempered to a certain point (temperature and time) which pins all the dislocations so they canít move. If they canít move, they canít absorb sound energy so that once struck, the bell rings for a long time. Bells are not made out of pure copper, in which dislocations move easily. But they are made out of alloys such as brass (Cu-Zn) or bronze (Cu-Sn) and then are carefully tempered. Suitably distributed impurities (alloys) can also stabilize dislocations.
Simply put, softer materials absorb mechanical energy, muting sustain, ambient string noise, and the high end/treble frequencies. Harder materials tend to increase sustain and volume and emphasize the higher/treble frequencies, including string noise.
Noise & The Blues: The Spirit of Life
Like the grit in Ray Charlesís voice or the soaring texture of Aretha, the textural sounds of the slide can convey emotion in ways that a clean, Hawaiian sound wonít. And vice versa. Like any good singer, the noise in the signal is there to be used as an expressive element. But to be useful, there has to be a contrast. If itís noisy because you canít control the sound, well, thatís not an artistic choice, níest-ce pas?
Iím a fan of slide playing with and without the noise. But when it comes to string noise and the shake, rattle and hum of the sound, I want to control it, rather than have it control me. Eliminating the noise all together as a stylistic choice, actually removes you from the Blues tradition, in the same way that an opera singerís version of ďR-E-S-P-E-C-TĒ is removed from the R & B, Gospel and Blues traditions.
To sum up, for acoustic guitar, you want something with higher mass in the middle of the harness range, that fits snugly on your little finger.
I played with heavy brass slides for years. I liked and played with the Acousti-Glide slides. Iím now really enjoying a hand-blown glass slide made for me by glass artist here in Vermont. I also like the porcelain MudSlides, which behave very much like bottle glass.
When recording, I often beef up the first and second string gauges on the guitar for recording slide parts, and have occasionally re-gauged the entire guitar to equalize the string tensions for a particular tuning.
Your comments are welcome Write me at firstname.lastname@example.org.
See also my pdf on "Rudiments of Slide: Left Hand Posture" which discusses and shows proper hand position necessary for the muting techniques critical to controlling the sound of the slide.