Mixes containing cement must have cement added immediately prior to use, and must bе used up within two hours of mixing. Calcium sulphate must bе used within 1 to 2 hours or lees of mixing, according to type, and particular care is needed to ensure that tools and mixing boards are kept clean and that partially set material does not become mixed with fresh plaster. Whenever a mix containing lime or cement is used as an undercoat time must he allowed for it to dry out and shrink before a further coat is applied. The finishing coat is known to crack otherwise with gypsum plasters it is only necessary to wait for the material to set reasonably hard before another coat can follow.
There are some factors in design which have some effects on the method of application, especially of renderings. If rendering is to bе applied to a large area of wall, the work cannot be completed as a continuous operation. There is, therefore, the difficulty of obtaining a satisfactory joint between one day's work and the next. Also the larger the area of work done as a single operation the greater is the risk of cracking. For both reasons there is much to bе said for designing the rendering as comparatively small panels and arranging some division between them. Whenever possible the lines of these divisions should coincide with places where cracks are likely to develop, for example, above and below window openings.. Another design point which must not be overlooked, is the difficulty of making good any rendered work after initial application. All holes for pipework should be completed and pipes be fixed before the rendering is done. It is also essential to fill in any holes from scaffolding before rendering. It is sometimes necessary to use a scaffold which is independent of wall support if a really good job is required. These points about the difficulty of making good rendering refer to renderings put on by trowelling. Plastic tubes are known, to be used for waste pipes where hot water discharge is under low pressure, and the small quantities of hot water that pass through such pipes do not harm it. Waste traps are now available with accessories, and there is a complete range of fittings and tubes for industrial uses with corrosive liquids or for installation in corrosive atmospheres.
EXERCISES.
I. Analyse the functions of the "ing" forms.
II. Find out verbs of obligation and give their equivalents.
III. Find out sentences with "Complex Subject", pay attention to the way of translating.
IV. Form derivatives of the following words:
hydrate, shrink, operate, satisfy, depend, еssenсе,
V. Answer the following questions:
1. How must cement be added to the mixes?
2. What precautions are nessesarу during the mixing of plasters?
3. What must be done when a mix is used as an undercoat?
4. What factor affect the method of application of rendering?
5. What should be done with the holes and pipes before rendering?
РAINTS AND PAINTING
General.
The term "paint" is taken here in the widest sense and it covers paint and all similar surface finishes. The purpose of painting mау be either for decoration or for preservation. Quite frequently both requirements have to be met.
Painting defects are by no means uncommon. They may arise from a variety of causes but in almost all cases they can bе classified as falling into one of three groups:
1) An incorrect choice of paint for the particular conditions - especially an incorrect choice in relation to backing materials.
2) An application of paint to a damp material, or a material to which moisture will have access.
3) Poor workmanship.
By suitable variation of the type and proportions of the ingredients, paints can be made glossy or matt as desired, or can bе made impermieable to water.
The types of oil paint which give a highly glossy finish are usually, called hard gloss paints or enamels. Flat oil paints give a matt finish.
Oil varnishes consist essentially of drying oil into which a proportion of resin has been incorporated by heating. The oil maу be a natural vegetable oil such as linseed, or sometimes a synthetic drying oil. Similarly the resin may be natural or synthetic. Apart from their use in paint manufacture varnishes are used to give a transparent glossy finish. Spirit vanishes are quickly drying vanishes made by dissolving resin in alcohol. They dry very quickly and give a glossy but somewhat brittle coating.
Water paints consist of pigments suspended a watery medium or in an emulsion of oil, resin от bitumen. They include distempers and color washes.Distempers are a type of water consisting no oil other organic matter and set primarily on Portland Cement. They are thus compatible with cement surfaces and provide an answer to the difficult problem of decoration on cement products. Cement paints give a matt finish and are only available in light shades. Grades made for external use offer a fair degree of water proofing. Bituminous paints of the solvent type are based on solutions of bitumen, often with a proportion of drying oil, in an organic solvent. They are used for waterproofing and the protection of iron and steel.
EXERCISES
