Understructures include the support system of webs, springs and fills. The understructure has several functions. It provides comfort for the sitter, it takes the weight of the sitter, it enhances the design through the lines of the frame, it prevents loose covers from slipping off and it holds the shape of the object while in use. Examples of different structures are illustrated in Figure 3.8. Fills are shown in Figure 3.9. These features, and springs, are also beautifully illustrated in James (1990).
Individual materials and upholstery structures have to withstand heavy wear over long periods of time. The higher the grade the more durable the product. In top-grade work, materials of the highest quality would be used. These include, for example, curled hair, linen cloth and webbing of flax, cotton, jute and hemp. Attention would also be paid to detail (historical correctness in profile and fabric, trim, seam construction etc.). Such work will last well but can involve much hand work. In low-grade work, materials would be of lower quality and might include coir fibre, Algerian fibre and, in particular, foamed plastics, jute cloth, textile waste, adhesives and poor quality staples. Such work will not stand up to heavy use. Lower-grade work may involve use of preformed units requiring less skill to fit. Many commercial mass-produced pieces are of poor quality - but not all. The main concern for such pieces is to look good from the outside; long-term durability is not a major concern.
There follows a survey of the most commonly extant materials and structures, however, if a material was locally available, economically viable and practical, it may have been used in upholstery. Pine needles, thistledown, leaves, bracken and chaff have all been used but are less resilient and therefore less likely to survive than more resilient materials such as curled hair.
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