The operation of pressing, i.e. the use of physical, pressure, combined with heat or heat and moisture, forms an essential part of garment maintenance. It is a form of temporary setting whereby fabrics are either set into a crease or set flat and unwanted creases are removed. Heat or heat and moisture, prepare fibers for the physical pressure exerted by the sole of an iron or the plates of a press. All the physical factors in pressing must be controlled to prevent either outright damage to the fibers, or unwanted effects on the fabric and it is perhaps better to examine the factors individually although they do not act in combination. Heat: All textile fibers are affected by heat in some degree. The natural cellulose fibers linen and cotton have a very resistance and can withstand higher temperatures than any other clothing fibers although they will ultimately 'yellow' and scorch if the temperature is consistently raised. An important factor in conjunction with heat is time. A prolonged exposure to heat is much more damaging to a fiber than a short one. In pressing of any kind there is heat loss and a time lag between the passage of heat from the source, i.e. the iron sole or the plates of the press, to the fabric. The longer the fabric is in contact with the source of heat the higher is its temperature raiser and if the source is heated to a temperature above the limit for the fabric, prolonged contact will damage the fabric, but a short contact will probably have no harmful effect because of heat loss between the surfaces the fact that the fabric is not in contact long enough for the temperature to build up to a dangerous point. HEAT AND MOISTURE Steam irons and steam presses, or the use of an ordinary iron and a damp cloth combine moisture and heat. Steam presses are supplied with steam under pressure, usually about 4.5kg per square cm. This gives a theoretical temperature of about 145*c but the heat lost as the steam heats the top and bottom press surfaces brings the temperature down to 100* to 110*c. This temperature is safe enough for most fibers except acrylic fibers where a lower temperature than 100*c region which again is safe enough for most fibers except acrylics. Cellulose acetate can be delustred in advertently by injudicious heat and moisture treatment and acrylic fibers should never be treated with heat and moisture together. Certain types of resin glaze are also sensitive to heat, or to hear and moisture because of the thermoplastic nature of the resin and these types of fabric and indeed any type of fabric of which the reaction is in doubt, should be tested on a spare piece before a garment is treated.