first to prevent absorption of the colours into the paper. In the past, photographic lantern slides were often coloured by the manufacturer, though sometimes by the user, with variable results. Usually, oil colours were used for such slides, though in the collodion era – from 1848 to the end of the 19th century – sometimes watercolours were used as well. Crayons and pastels The use of crayon or pastel sticks of ground pigments in various levels of saturation is also considered a highly skilled colourist's domain, as it requires knowledge of drawing techniques. Like oils, crayons and pastels generally obscure the original photograph, which produces portraits more akin to traditional paintings. The Photo-crayotype, Chromotypes and Crayon Collotypes were all used to colourize photographs by the application of crayons and pigments over a photographic impression. Charcoal and coloured pencils are also used in hand-colouring of photographs and the terms crayon, pastel, charcoal, and pencil were often used interchangeably by colourists. Hand-coloured photographs sometimes include the combined use of dyes, water-colours, oils, and other pigments to create varying effects on the printed image. Regardless of which medium is used, the main tools to apply colour are the brush and fingertip. Often the dabbing finger is covered to ensure that no fingerprints are left on the image. Preservation and storage In general, the preservation of hand-coloured photographs is similar to that of colour and monochrome photography. Optimal storage conditions include an environmentally controlled climate with low relative humidity (approximately 30-40% RH), temperatures under 68 degrees Fahrenheit (20 degrees Celsius), and a low concentration of particulate pollution, such as sulfuric acid, nitric acid, and ozone. The storage area must also be clean and free of pests and mould. Because hand-coloured photographs, like colour photographs, are more sensitive to light and UV radiation, storage should be in a dark location. The storage area should be secure and monitored for internal threats – such as change in temperature or humidity due to HVAC malfunction, as well as external threats, such as theft or natural disaster. A disaster plan should be created and maintained for all materials. When handling cased photographs such as daguerreotypes, albumen prints, and tintypes, especially ones that have been hand-coloured, caution is required. They are fragile and even minimal efforts to clean them can irreparably damage the image. Hand-coloured cased photographs should be stored horizontally, in a single layer, preferably faced down. Cases