WARPING OF WOOD

Press, Volume LXIII, Issue 19166, 24 November 1927, Page 4

WARPING OF WOOD

PROBLEM OF PREVENTION. RESULTS OF TESTS. The problem of preventing the warping and twisting of wood under varying conditions of moisture is a serious one for all users of this important material. Recently the Forest Products Laboratory of the United States Department of Agriculture has undertaken to study this problem carefully, and some of the results of tests of various protective coatings are reported by M. E. Dunlap in "Industrial and Engineering Chemistry." The tests made were conducted with the idea of determining the valuo of different types of wood coatings in preventing' warping of test specimens under various moisture conditions. The conclusions reached are summarised by Mr Dunlap as follows: "Linseed oil is commonly considered to be an excellent moisture-proofing material, but we have not found it so. We have tested it in many ways—by brush coating, dipping, soaking, and by impregnation under pressure. "Wax coatings similar to those used on floors do not rate high in moistureproofing value. "While we have not made a detailed investigation of the effects of various constituents of coatings, some rather significant points are brought out in our studies. Spar varnishes are low in moisture resistance as compared with rubbing varnishes. For instance, tho officieney of spar varnish, when new, averages about 57 per cent., and that of rubbing varnishes 88 per cent. Apparently the greater amount of gum present' in a rubbing varnish accounts for the difference.

' "The addition of a pigment to a varnish or oil increases its efficiency in keeping out moisture. An oil paint is far more effective than linseed oil alone, and an enamel shows a marked improvement over varnish. Aluminium Powder. "Aluminium powder is very effective in keeping out moisture when used in the preparation of a coating. When freshlv mixed with the vehicle it leafs out, "forming a very effective barrier against moisture changes. Probably the most striking illustration of its value is in raising the efficiency ot bronzing liquid on first exposure from about V2 per cent, to 92 per cent., a gain of 80 per. cent. Addition of aluminium powder also increases the efficiency of spar varnish by about 40 per ceiit. From one to two pounds of powder are required to the gallon of the vehicle. "The majority of asphalt and pitch paints have good moisture resistance. Their efficiency, however, may also be improved bv the addition of 10 to 2U per cent, df aluminium powder. In some cases tho powder will leaf out 'and rise to the surface, hiding the black colour. "Probably the most effective protection against moisture changes is the aluminium leaf coating. This process simply incorporates a layer of aluminium leaf in the coating and provides a very strong harrier to moisture. The efficiency of this coating is about 98 per cent. It was developed particularly for use on airplane propellers, where very slight changes in moisture content are important. Other Preventives.

"Cellulose lacquers have an efficiency of a little over 70 per cent, when new, and cellulose enamels about 5 to G per cent, higher. "Shellac has a remarkably high moisture-proofing value of about 88, per cent., but docs not stand up very well when subjected to the weather. "It is important to know that none of our ordinary coatings is absolutely water-proof. All permit tho passago of moisture to some degree, and if extreme conditions persist over a considerable period of time, injury may result to glue joints and woodwork which thev cover." In conclusion, Mr Dunlap adds a precaution to manufacturers who seek to prevent warping when applying a coating to me side only of the article to be protected. Ho says:— "Few manufacturers realise tho effect that coatings may have on warping. In many kinds of goods one surface of a panel is .finished with a very effective coating, while ho attention is paid to the back. When moisture changes take place the gain or loss of moisture is greater from the exposed side than from the coated side, and if the changes occur rapidly, warping is almost sure to result. Such difficulties may bo avoided by applying to the backs a cheap coating which will balance the face coating in moisturo resistance. Such practice would reduce tho rate of change in moisture content and permit better equalisation of absorbed moisture." BLOCK OF OFFICES. i ■ NEW BUILDING FOR HEREFORD STREET.

A permit was issued yesterday for tho erection of handsome new offices, in Hereford street, for the National Insurance Company. To provide a site the company's present offices will be demolished, and the new building, which will have four storeys, will be of the same height as Hereford Court, tho adjoining structure. On the frontage to Hereford street, the newedifice will have a width of 31ft. Iron window balconies, and four pilasters with lonic capitals will embellish the frontage, which will be cement-faced in light biscuit colour, the building itself being of reinforced concrete with stvol joists. On the parapet, which will be surmounted by the company's coat-of-arms (shield, with lion) will "bo j four steles symmetrically placed. The flat concrete roof will havo a cover- I ing of reinforced malthoid. The two large windows on the ground floor will havo semi-circular tops, similar to that of the main entrance, which will be an ornamental iron gate, with grille. Rusticated jointing on this floor will further enhance the dignity of the building. The plans provide for marble panelling and dado eight feet high on the ground floor, and white marble pavement for the porch. Most of the accommodation on the ground floor will be utilised by the company, but a small suite of offices will be available for letting. A ramped way will lea;! to the strongroom and basement.

Each of the floors above will have suites of offices available for tenants, a strongroom, and lavatories, and a lißht area will be let into the building. An up-to-date heating system and lift will be included in the modern amenities for the convenience of those in the building.