I've just heard of "silk and steel" strings, but can't find out what they are. So, what are they made out of? Do they produce a significantly different amount of tension in the neck?
Silk and steel strings are made to be used on a steel string acoustic guitar. They no longer use silk but instead use a nylon filament that resembles silk.
These strings will not work well on an electric guitar because the windings are usually a bronze alloy which will not respond to magnetic pickups (the way the nickle windings on electric strings do) and the steel core is shielded by the windings and the "silk" to the extent that they are virtually invisible to the magnets.
The wound strings on a set of "silk" and steel strings have a layer of nylon filament between the metal outer winding and the steel core. The plain steel strings are no different than any other electric or acoustic plain steel strings. See the pictures below to see the difference between silk and steel strings, normal acoustic strings, and normal classical nylon core strings.
The main characteristics of "silk and steel" are as follows:
1) Because the nylon filament acts as sort of a padding - they have a much softer feel than regular steel string acoustic strings. This makes them easier on the fingers and many guitarist who play fingerstyle using the meat of their fingers instead of fingernails or finger picks prefer the softer feel. The softer feel is also more finger friendly in general for both the picking fingers and the fretting fingers.
2) The sound is much more mellow than normal steel strings due primarily to the damping effect of the "silk" wrapping. Some players prefer the more mellow sound.
3) They tune up with lower tension than normal steel string wound strings. This is because the wound strings have a thinner steel core to allow room for the "silk" wrapping between the outer winding and the steel core. The plain steel strings (b and high e) tune to the same tension as any steel string with the same gauge but they tend to be on the lighter side in a set of silk and steel strings. Warning - the tension is still too high for a classical guitar with no truss rod.
4) The lower tension contributes to the softer feel because you don't need to press as hard to get the string to the fret. Because of the lower tension they are easier to bend. Although most folks who use silk and steel strings are playing more classical style music with fingerpicking and don't often bend strings. Also because of the lower tension - they are not well suited for heavy strumming because the string deflection will be greater than higher tension normal steel strings.
5) They are quieter to play because string noise caused by fingers sliding on the strings are muted somewhat by the "silk" wrapping. This might be an advantage for fingerstyle players and a big advantage for players who use sliding as part of their technique.
Silk and Steel strings are ideal for fingerstyle players who prefer to play a steel string guitar and want a mellower sound and softer feel - similar to a nylon string guitar.
Silk and Steel strings are generally made to have lower tension and a mellower tone than regular steel strings. I don't know if all "Silk and Steel" strings are made from the same materials, however, or even necessarily contain silk.
- GHS says its Silk and Steel strings have a silk and steel core wrapped with silver-plated copper wire. According to GHS, this results in a lower-tension string with a mellower tone than the equivalent standard steel string.
- Martin says its Marquis Silk & Steel Folk strings are made "...with tin-plated steel core wire and silver-plated copper compound wrap wire" according to Martin. If you look at the tension chart on the referenced page, you'll see that the Folk strings have lower tension than either the 80/20 bronze or the 92/8 Phosphor Bronze strings.
Also, just to further confuse the issue of what "Silk and Steel" means, Martin says;
We created our Marquis line to pamper your guitar by adding a soft silk wrap to the ball ends which prevent wear and tear on the bridge and end plate as you play. Our silk wrap puts a comforting layer of protection in place every time you change your strings.
I have a set of Marquis Fok Silk & Steel strings on my acoustic. They are very easy playing with a nice mellow tone, but also not very loud and easy to "accidentally" bend if you use too much pressure.
It's what you would use on a steel string acoustic guitar to make it sound like a classical/nylon/Spanish (all the same thing) guitar. I used to work at a music store and this is what they were for. They give a mellow sound. You can't use nylon strings on an acoustic guitar made for steel strings because there's not enough tension on the neck once the strings are tuned up. So the silk and steel let you have a nylon string guitar sound on your steel string. Course, they don't sound exactly like a nylon string, but they do pretty good. Also, because they have a steel core, you can hear them with pickups. Hope this helps. Peace.