HARDWOOD RECORD 



23 



tree was cut down in the garden of Eden. There have been varia- 

 tions in the method of sawing, the method of piling and the length 

 I time the lumber has been exposed to the various climatic cou- 



itions of the different locations. There is no denying the fact 

 that the process of air-drying is slow and fairly .safe, but always 

 accompanied by discoloration, warping and surface and end check- 

 ing. This is caused when the surface dries more rapitllj' than the 

 interior and a tension is set uji wliich produces internal strain 

 that either checks the outside of the lumber or honeycombs the 

 ■ enter, e. g., oak will always have the surface checked in stock 

 one inch, or show honeycomb in two, three or four-inch thicknesses. 



Other classes of woods, such as gum or pine, that grow in the 

 moist climate of the South, will show mildew or a fungous growth 

 of a rather bluish color and mottled character, which begins im- 

 mediately when an attempt is made to air-dry the stock and which 

 seriously reduces its valuc.^ The bluish color is always consid- 

 ered a selling defect in th^ yellow pines of the South, even when 

 used for structural purposes. It is therefore evident in woods of 

 this character that air-drying has its serious disadvantages. 



The first attempt at artificial drying of lumber was the smoke 

 kiln which may be remembered by the old woodworkers of the 

 ]iresent time. Its original form was that of a collection of boards 

 standing over a fire built in a hole in the ground. The boards were 

 placed on end, wigwam style, and the heat and smoke were sup- 

 jKiscd to dry the lumber. The smoke kifn in later form was a series 

 of brick arches in which fires were built and maintained by burn- 

 ing sawmill waste. The smoke and hot gases from these fires 

 passed through the lumber in the dry kiln and were supposed, by 

 means of heat, to evaporate and carry away the moisture in the 

 lumber. It is still used to a limited extent in out-of-the-way 

 ] 'laces where modern progress has not affected the industry 

 to any marked extent. The lumber dried in the smoke kiln was 

 scorched and always discolored by smoke, and the fire hazard of 

 this smoke kiln was so great as to make it uninsurable. The 

 process was slow, the cost of operation enormous on account of 

 excessive labor required to tend the fires, and the resulting lumber 

 was dried so unevenly and by haphazard that satisfactory results 

 were never obtainable. A certain amount of moisture was re- 

 moved which reduced the shipping weight, but other than that, the 

 result of the smoke kiln was very unsatisfactory. 



The next type in the development of the kiln was the bake-oven 

 kiln. This type has numerous varieties, but in every case involved 

 the introduction of heat into the kilns by means of steam in some 

 form of radiators or pipe coils. The air in the kiln passing over 

 the coils or radiators and circulating in through the lumber was 

 supposed to vaporize the moisture in the boards. The ventilation 

 in these kilns, and the circulation of heat in the same were matters 

 that received little or no attention, and, in fact, were even less 

 appreciated. In any event the improved results of the bake-oven 

 kiln over that of the smoke kiln previously known were so marked 

 that the enthusiastic advocates of these steam radiation kilns 

 thought perfection had been atttained. 



It is of cour.se obvious that in the bake oven kiln there was 

 little or no control of drying conditions, except such control as 

 might come from the regulation of the heat when more or less 

 steam was admitted through the valve to the radiator or coils. No 

 attempt was made to introduce or control humidity, and kilns of 

 this type were usually built with no consideration for the build- 

 '":; material used, as to its durability, quality, tightness or 

 I -Illation. 



It is therefore evident that the atmosphere in a bake-oven kiln 

 would usually be very dry and that there would be a serious tend- 

 ency to heat the lumber too quickly after inserting it in the kiln. 

 It was inevitable that lumber dried in this type of kiln would be 

 badly rrusted or casehardened. In other words, a shell of one- 

 eighth to one-fourthinch thickness would be formed' by the heat 

 on surfaces and edges of the board, and the center of the board 

 would be not appreciably affected by the heat, nor would the 

 center moisture be removed to any extent. We therefore have an 

 operation which is very similar to broiling beef steak, in which 



the attempt is made to sear and seal the juice in the meat. 



It is evident that this was exactly wrong in lumber and the 

 moisture left in the center would eventually cause difficulty either 

 by checking, honeycombing, warping, winding or twisting, and 

 would ruin any fine piece of woodwork. Another very good exam- 

 ple of crusting can be taken from the domestic experience of 

 baking biscuits or potatoes. It is a well known fact that when 

 they are placed in a hot oven of the kitchen range or gas stove 

 there is a crust formed on the exterior which seals the moisture in 

 the center, and the housewife will testify to the fact that the too 

 rapid baking of a biscuit or potato will leave a soggy center. 



Another feature of this uneven drying, which makes bake-oven 

 dried lumber unsatisfactory for commercial use, is the fact that 

 lumber is over-dried on the outside and under-dried inside and is 

 always liable to change shape when it is planed, dressed, ripped 

 or resawed. The bake-oven kiln, therefore, has disadvantages 

 which are rapidly relegating it to the scrap heap, in the same way 

 that the old side wheel steamer is giving away to the more modern 

 stern propeller. 



A modification of the bake-oven kiln that is in more or less use 

 today is the so-called blower kiln. This type of kiln offers more 

 possibilities of circulation than its predecessor, but it is at the 

 same time to be equally criticised for its exterior easehardening, 

 checking and interior honeycombing. It is perhaps easy to see 

 that in a blower kiln the rapid movement of so much air, as accom- 

 plished by the blower process, will inevitably tend to create an 

 excessive and extravagant fuel bill. 



In other words, the air, which is forced rapidly over the heat- 

 ing coils in a blower kiln, will be moving at high velocity and 

 will be very thirsty or have a strong affinity for any form of 

 moisture. This will have two results: First, the air, because of 

 its velocity, will absorb only part of the moisture and do only 

 part of the work of which it is capable, meaning extravagance in 

 heat; and second, the air surrounding the lumber will always be 

 very dry, which will cause even more serious easehardening than 

 mentioned in the bake-oven kiln. The blower kiln has practically 

 no control either of temperature or humidity. 



The blower kiln, because of its rapid absorption, however, has 

 a certain value in connection with the rapid surface drying of thin 

 veneers. The proper means of humidifying and control are still 

 lacking and an economical development along this line may be 

 expected. 



During the last ten years there has been a rapid change of 

 front on the part of those interested in securing scientific and 

 accurate results in the use of lumber dry kilns. The rapid change 

 has been due perhaps partly to the general movement all over 

 the country toward a keener public consciousness and partly to the 

 more extended application of practical chemistry and physics to the 

 woodwork manufacturing trades all over the country. The modern 

 efficiency expert is another indication of the progressive spirit. 



The first modification in lumber drying was the appreciation 

 of the fact that something must be done to decrease the costly 

 easehardening and its attendant evils. A less dry heat was thought 

 advisable, and it w-as not long before every manufacturer of dry 

 kilns was talking "moist air kilns." What was or was not 

 covered by the term "moist air" was hard to discover. Some 

 enthusiasts went so far as to boil their lumber, and from this 

 extreme some sawmills went to the opposite point and introduced 

 an open end one-fourth-inch 12.3-pound pressure live steam pipe 

 into the dry kiln. The result of the attempt to introduce moisture 

 in connection with lumber drying was, of course, beneficial and 

 there was a consequent reduction in the amount of easehardening. 

 Almost coincident with the discovery of the need of moisture was 

 the attempt to shorten the time of drying. It was perfectly 

 natural to conclude that as moisture decreased easehardening it 

 would permit quicker drying. The eagerness to hurry practically 

 offset the improvement caused by moisture and the aggregate result 

 was comparatively little actual progress. There was, therefore, a 

 woeful lack of understanding of another fact — that the more 

 rapidly you dry lumber the more carefully you must control the con- 



