American felts are fabricated from sheep’s wool by a method which utilizes the natural characteristics of the fibre to produce “an interlocking, self-tightening fabrication” in a wide range of consistencies.
Compression, moisture, heat and agitation are the conditions required for hardening and fulling wool into felt.
Upon the degree of fulling depends the density of the finished felt, by which its several valuable properties are determined.
Felt may be defined as a fabric composed basically of sheep’s wool, formed by the interlacing of individual fibres in all directions. It has a grainless structure varying from a loose, springy mat to a solid substance of rock hard consistency and having a range of physical properties dependent on density. There are many different ways in which fibres may be combined to form cloth, but the felting process is the only one in which the natural qualities of the raw material are used to the greatest advantage, this is, with the least possible alteration. Sheep’s wool in its natural state had nearly everything required of an ideal fabric for the uses of mankind. What it lacks, the felting process seeks to supply without destroying any of the original values.
The shorn fleece itself is a kind of loose fabric, but greasy, soiled, matted and uneven in texture. When the wool has been thoroughly cleaned, graded, combed out and recombined by a method which simulated the original random trend of the fibre but tightens their embrace, felt is produced. True felt is in effect a reconstruction and improvement of the fleece. In distinction from all other manufacturing operations, felting must be thought of as a physical process which employs the horn-like cellular structure of the hair filament without subjecting it to permanent change. Such a method is sharply in a contrast with the chemical processes which, as in vulcanizing rubber, or “setting” thermo-plastics, involve a reorganization of molecular structures. It also differs radically from the textile processes in which fibres are twisted into yarns which are crossed in weaving, thus subjecting them to persistent unnatural tensions.
In felt, the fibres are recombined without being systematically distorted. Their natural crimp is retained and advantage is taken of this property of the wool to stimulate the tangling tendency of the fibres which is a prominent natural characteristic of the raw material. The general method of manufacturing felt consists in agitating a prearranged mat of fibres under compression, while in a warm, moist condition which best promotes inter-fibrilar movement. This causes the individual filaments to travel into a mutual entanglement which becomes closer as the process is continued. The effect is to produce a closer structure which has often been referred to as an “interlocking, self tightening fabrication.”
The longer the process is continued, the tighter becomes the embrace of the fibres; hence the greater the number of fibres occupying a given volume; therefore, the greater the strength of the resulting material. In preparation for felting, the wool must first be freed of burrs and other foreign matter and scoured to remove the natural grease. It is then carded into a web in which the fibres are drawn into substantial parallelism. Successive webs are piled one upon another to form batt in preparation for the felting operation. The webs are gossamer tissues having a certain amount of longitudinal strength, due to the overlapping of the longer fibres of which they are composed. By crossing alternate layers transverse strength is imparted to the pile, which is thus sustained in length and breadth only, but is still composed of loose layers of the carded wool, having only slight laminar adhesion.
Consolidated of the batt into a fabric having interfacial strength is effected by a process distinctive to this material, known as hardening. It is this step which causes the entire mass of material to be agitated while under pressure and in a warm, moist condition. This induces movement among the individual fibres whose elasticity causes them to seek to regain the natural crimp of which they have been temporarily deprived by the carding. A traveling, piercing action in the direction of the root end of the fibre is involved, inducing penetration of the fibres from one web to the next, bonding them together. In this way a soft fabric of loose texture is produced, harder than the preceding batts, having some strength in the directions of length, breadth and thickness. This is the first control of the three dimensions and changes only that of thickness.
A second stage in felting is required to “tighten up” the interlocking mechanism which has been formed in the hardening process. This step is known as fulling, and it is readily described as a controlled process of shrinkage. In principal, fulling is a process of shrinking. It involves the application of pressure to a mass of hardened felt, while in a warm, moist condition. As the density of the material increases the friction between the more closely compacted fibres is increased, so that to continue felting, impact is applied under simple pressure. The hardening operation is effected in a platen in which provision is made for a rapid vibration. Fulling is carried out in a hammer mill which kneads and turns the material with the application of considerable pressures. Both operations are required to produce felts to the consistencies required for most purposes and, in general, the degree of hardness or density is a function of the total fulling time, as the longer the felt is fulled the harder it becomes.
The peculiarity of the fulling action is that it causes a shrinkage in length and breadth of the piece, together with an increase in thickness, due to the automatic rearrangement of the fibres under the hammering action of the mill. As this action progresses, the laminar structure of the webs gradually diminishes. Third dimensional strength increases, as measured by splitting resistance in the end product. Eventually the granular form disappears entirely, leaving a practically solid fibrous or more nearly granular mass. All of this is accomplished without the use of artificial binders or modification of the hornlike structure of the wool fibre.
True felt may thus be fabricated from sheep’s wool along, in a complete range of textures, density and strength. The use of stiffening agents to produce certain effects, or chemical treatments, such as flameproofing, verminproofing, mildeproofing dyes and other impregnants or coatings, when specified, is supplementary to the felting process and independent of it. The construction of the material is completed in the fulling mill. After fulling, the felt must be washed and neutralized to remove traces of chemicals used in the wool scouring, soaps or weak acid, which are used as lubricants or to soften the fibres during processing.
Colored felts go through the dye-house before completion. The final manufacturing steps are dying and finishing. After centrifugal extraction of the rinse water, roll felts are dried under tension to remove wrinkles produces in the mill. Sheet felts, which are frequently the thicker and harder products of heavy fulling, are oven dried without tentering. Finishing includes trimming to uniform width and pressing, shearing or grinding the surfaces to specified thickness.