A common innerspring mattress consists of three components:
Upholstery layers, which encase the mattress core.
The spring core, also called innerspring unit.
The foundation, which many people call box spring
Spring Mattress Core
The core of the mattress is the most important part in the support of the body. In most modern mattresses (except for the foam-only ones), it is made up of steel coils springs, also known simply as coils. The spring mattress core is also called innerspring. It provides support to the set. Proper support is where the wire coils push back in response to weight deflection to hold the body in alignment. The steel coil spring was invented during the industrial revolution and was first patented for use in a chair seat in 1857. Then, in 1871, Heinrich Westphal invented the innerspring mattress. He lived in Germany and died in poverty, having never profited from his invention.
Number of coils
In the past, conventional wisdom said that the more coils a mattress has, the more support it will give the user. This is not necessarily true today. The coil construction will determine the amount of support the user gets. Generally, but depending on the mattress size, they come with 300 to 800 coils. Depending on the size, 500 to 800 coils is considered firm. However, some expensive and comfortable mattress only have around 400 coils, and it takes as few as 312 coils in a full mattress to provide decent support. This is due to the use of a heavier gauge wire, resulting in more steel. Assuming that the overall coil count is decent, a heavy duty innerspring mattress may also increase the longevity of the mattress for large users.
Additionally, there are some other misconceptions as far as coil count and firmness are concerned. Doctors once said that a firmer mattress will give better support. This was actually only true with a small percentage of people, back and stomach sleepers. Most people sleep on their side, and as the side of the human body is not flat like the back, better body conformity provides better support because it put the spine into proper alignment or it's natural resting position. Usually this is achieved with an individually wrapped coil system (see below.)
Gauge of the coils
Most would think that obviously heavy gauge (thick) coils will offer a great deal of support while light gauge coils will give less support. This couldn't be further from the truth. Some mattress manufacturers will compensate for a higher (thinner) gauge of steel by using a higher coil count. A low coil count and heavy gauge wire can pass as a firm mattress in the store, but it is likely that this mattress will become lumpy quickly because it does not have enough coils. Many premium mattresses feature 14-gauge (1.63 mm) coils. Coils are measured in quarter increments. The lower the number, the thicker the spring. With coils of 14 to 15.5 gauge (1.63 to 1.37 mm), it is important that the total coil count be high to compensate for the fact that they give so easily under pressure. A 12.5 gauge (1.94 mm) innerspring, the thickest typically available, may feel rock hard in a double mattress even with a coil count of 400 or less.
Connections between the coils
Generally, the number and quality of these interconnecting wires is not published by the manufacturers. If there are too few of these interconnecting wires a mattress can lose its shape more quickly than one that has an adequate amount. Mattresses that sag quickly usually have few interconnecting wires. There are some manufacturers that make individually pocketed coils, which avoid movement on different sides of the bed because they are not interconnected. However, the special pockets provide the support required by the coil instead of the interconnection with other coils.
Types of coils
Construction-wise, most manufacturers claim to have the best coil-to-gauge ratio. However, there are only a few innerspring manufacturers and four general types. One thing to remember is that while there are many different types of coils and many manufacturers, most manufacturers have their coils made by one company, Leggett & Platt.
Bonnel coils
These are the oldest and most commonly used. They were adapted from buggy seat springs of the 19th century. They are still prevalent in low priced mattresses. They have an hour-glass shape, and the ends of the wire are knotted or wrapped around the top and bottom circular portion of the coil (round-topped and self tied).
Marshal coils
Also called "pocketed coils." Each coil is wrapped in a fabric encasement, and is usually tempered twice, once with heat and once with electricity. The brand Simmons uses pocketed coils in their mattresses. Simmons also does some things that other brands that use pocketed coils don't. They "pre-compress" their pocketed coil, which means that they pack the coil into a "pocket" that is shorter (usually by a couple inches) than the coil. By "pre-compressing" the coil, it becomes firmer (sort of) because the coil is already in action (it is always pushing up.) Additionally, when two people sleep on the same mattress, the individually wrapped (or individually pocketed) coil provides motion separation (when one person tosses and turns, the other doesn't feel it.) This is important because when one person tosses and turns, it may not completely wake the other person, but, it can take them out of a deep sleep which is when the body does all of its regeneration. When a sleeper is undisturbed at night they can fall into that deep sleep faster, stay in a deeper sleep longer and wake up feeling more refreshed because it allows the sleeper to get more use out of their sleep.
Offset coils
Bonnell coils evolved into Offset coils, which are currently popular, but happen to be the most expensive ones. They are almost identical to the Bonnell coils, except that the top and bottom circular part of the coils have flat or straight sides, which allow for better hinging action when they are placed together in a spring unit, and which can be easily aligned. An evolution of the offset coils brought the elimination of the knot that tied the end of the wire to the coil. In other words, offset coils might be tied off or feature a loose end (sensory arm). These are called "open end offset coils". These improvements gave offset coils an extra turn of working wire for greater spring action on each end of the coil.
Continuous coil
They look like double wire spirals or loose ringlets, creating a network of wire running nonstop through the entire bed. The brand Serta uses continuous coils in their mattresses. Kingsdown also uses such a system. The idea is that with a continuous coil system, each coil is connected to the next so that it can draw strength from it. This type of coil system does tend to be the strongest, however we must remember that strength does not dictate support.
Foundations
Although in the past the foundation actually contained springs, increasingly today it functions primarily to elevate the mattress, although a good box spring can help prolong the life and comfort of the mattress. There are three main types of foundations:
Traditional wood foundation
It is usually made of pine or similar hard wood. It usually has seven or eight support slats, covered with cardboard or beaverboard. This type of foundation, called by the industry a zero deflection unit , increases the feeling of firmness or stability.
Box spring
It features extra-heavy-duty springs. If the springs match the mattress it is called a coil upon coil box spring. Since a box spring has coils, it generally increases the give in the mattress, which in turn produces a softer or bouncier feel. Today, the boxspring is all but extinct. Research showed that boxsprings had too much give to them and caused coiled mattresses to sag prematurely. Most mattress manufacturers have switched to a rigid foundation with little or no fluctuation. By doing this and putting a high density block foam directly underneath the coils, the mattresses last longer.
Grid foundation
It is a combination of steel and wood.
Upholstery Layers
The mattress core is covered by several soft materials, providing cushioning and comfort. Some manufacturers call the mattress core by the name "Support layer" and the cushioning materials by the name "Comfort layer". The "Comfort layer"can be divided into three sub-layers — Insulator, Middle Upholstery and Quilt. Aside from the number and gauge of the coils, the upholstery layers are used to differentiate the different "qualities" of mattresses that manufacturers produce.
Insulator
This layer separates the mattress core from the upholstery, and it is usually made of fiber or mesh, with the intention of keeping the middle upholstery in place.
Middle upholstery
The middle upholstery comprises all types of materials on top of the insulator and beneath the quilt. It is usually made from materials that give maximum comfort — regular foam, visco-elastic foam, felt, polyester fibers, cotton fibers, convoluted (or egg-crate) foam, non-woven fiber pads, etc.
Quilt
The quilt is a top layer of the mattress made of light foam or fibers stitched to the underside of the ticking, and provides the immediate soft texture that the user feels when lying on a mattress. The quilt can be firm or soft and plush. The protective fabric cover, which encases the support and comfort layers of the mattress, is called ticking. It is usually made to match the foundation and comes in a wide variety of colors and styles. Most ticking is made of synthetic fibers like polyester, latex and acrylic, or of natural materials such as cotton, silk & wool. The ticking produces the look and feel of the mattress, so it is usually soft to the touch and attractive to the buyer (at least that is what manufacturers intend to create).
