Fabrics are often woven in lengths of 50m, 100m or more. Manufacturers and distributors then hold these lengths and at some point a designer buys a smaller cut-length. If subsequently the fabric is damaged when being made up or if the end-client has requirements for additional fabric, then more is ordered. So far so good? But exactly which roll is the additional fabric going to be cut from?
It is important to realise that there can be variation in the dyes used. Normally manufacturers keep a record of the exact dye(s) used in the manufacturing process(es). This is a dye lot. The dye could have been added to make a coloured yarn early in the process or it could have been added to the fibre later if it is a printed fabric.
So when more fabric is ordered for the same client it is important to ensure the same dye lot was used as for the original order.
Variations do occur in almost all fabrics. So if the same dye lot cannot be re-ordered it is prudent for the designer to order a sample for matching. It is wise for designers, upholsterers, curtain- and cushion-makers to always request a sample to avoid costly mistakes.
Typically man-made/synthetic dyes are more easily chemically replicated and so are inherently less prone to dye lot variation. For natural products there is a much greater chance of variation. But again there is no hard and fast rule.
Colour also plays a part as some farbic colours are more prone to change because of the chemical structure of the dye and/or the fabric being used.
What if all the stock has been used and there is a variation? Do we have to start again?
Hopefully not and common sense has to prevail from time to time. For example, a dye lot variation can be negated by conditions in the final installation. So in the case where there are fabrics from different dye lots with one dye lot only used in each room only rarely will the be any noticeable difference from room to room. Even in the case where, say, curtains on different walls are made from different but similar dye lots then any dye lot variation can be made unnoticeable by the effect of varying amounts of sunlight through windows. Although bear in mind there are variations in the strength and type of light throughout the day and between sunlight and artificial light at night. Similarly in low light (cellar) conditions dye lot variation will be much less noticeable.
Finally bear in mind the age of the fabric/curtains and the degree to which they have been exposed to sunlight. Some fabrics naturally fade over time, depending on their colour fastness. In this case it is unlikely ever to be able to get a perfect match.