TECHNICAL NOTE number B-M 
 
 UNITED STATES DEPARTMENT OF AGRICULTURE 
 
 FOREST PRODUCTS LABORATORY 
 SIN — REVISED 
 
 FOREST SERVICE 
 
 October 1956 
 
 METHODS OF 
 
 SPECIFIC GRAVI 
 
 DETERMINING THE 
 lAVlfTY OF WOOD 
 
 Next to actual strength tests, the specific gravity of wood "aff or-ds thejpest 
 indication of its strength properties. The specific gravity of any piece of 
 wood may easily be determined by the methods described in this note. 
 With the aid of table 1, the strength of the piece as a beam or as a col- 
 umn, its shock-resisting ability, its hardness or wear- resistance, its 
 toughness, its shearing strength, and its value in several other respects 
 may be estimated. 
 
 The specific gravity of a substance is its weight divided by the weight of 
 an equal volume of water. As both the weight and volume of wood vary 
 with the amount of moisture in it, specific gravity as applied to wood is 
 an indefinite quantity unless the circumstances under which it is deter- 
 mined are specified. The specific gravity of wood is almost always based 
 on the weight when ovendry, but the volume may be that in the ovendry, 
 air-dry, or green condition. The true specific gravity of wood is, of 
 course, that based on volume when ovendry. For greater convenience 
 inmaking determinations, however, the U. S. Forest Products Labora- 
 tory bases specific gravity on the volume of the specimen when tested 
 and has determined the relation of strength to specific gravity separately 
 for green and air-dry wood. 
 
 In using table 1 for estimating the properties of particular timber, there- 
 fore, it is necessary to determine specific gravity on the volume of the 
 sample in a green condition or at a moisture content of about 12 percent 
 (air-dry condition). The volume when ovendry may be obtained and con- 
 verted into the volume when green by means of shrinkage figures. 
 
 Specific gravity determinations may be mad 
 should contain no more than 25 cubic inches 
 siderably more time for drying. 
 
 ,"P^ p OTa£RY 
 
 MAY 11 • 
 
 I.F.A.S.- Univ. cf Florida 
 
After selecting a representative specimen, proceed as follows 
 
 To find specific gravity of wood 
 basedupon volume in a green or 
 air-dry condition 
 
 To find specific gravity of wood 
 basedupon volume whenovendry 
 
 1. Find volume of specimen by 
 measurements or by immer- 
 sion method. 
 
 1. Put specimen in oven at 103° 
 C. (plus or minus 2° C. ) and 
 dry until constant weight is 
 attained. 
 
 2. Put specimen in oven at 103° 
 C. (plus or minus 2° C.) and 
 dry until constant weight is 
 attained. 
 
 2. Weigh specimen. 
 
 3. Weigh specimen, 
 
 3. Find volume of specimen by 
 measurements or by an im- 
 mersion method. 
 
 4. Compute specific gravity, using formula 
 
 D 
 V 
 
 Specific gravity 
 
 where D = weight in grams and V = volume 
 in cubic centimeters. 
 
 When weights or measures are not taken in metric units, use the follow- 
 ing reduction factors: 
 
 Inches x 2.54 = centimeters 
 Cubic inches x 16.4 = cubic 
 centimeters 
 
 Ounces x 28.4 = grams 
 Pounds x 454 = grams 
 
 Both the ovendry weight and the volume should be correct to within at 
 least 1/2 of 1 percent. 
 
 Determining Volume of Specimens 
 
 Lineal measurement method. --The specimen must be regular in shape 
 with right-angle corners for determination of its volume by lineal meas- 
 urement. It should be measured carefully to determine its length and 
 
 -2- 
 
cross sectional area. Its volume is equal to the product of its length and 
 cross sectional area. 
 
 Immersion methods . --The volume of a regular or irregular specimen 
 may be found by determining the weight of water it displaces when im- 
 mersed. This weight in grams is numerically equal to the volume of the 
 specimen in cubic centimeters. 
 
 Method A . - -A container holding enough water to completely submerge 
 the specimen is placed on one pan of a balance scale. The combined 
 weight of the container and water is then balanced with weights added to 
 the other scale pan. By means of a sharp pointed rod, the specimen is 
 held completely submerged without touching the container while the scales 
 are again balanced. The weight that is added to restore balance is the 
 weight of water displaced by the specimen. 
 
 POINTED ROD 
 
 CONTAINER 
 
 WATER 
 
 SPECIMEN 
 
 WEIGHTS 
 
 METHOD A 
 
 Method B . - -A container holding enough water to completely submerge 
 the specimen is placed below one pan of a balance scale from which a 
 wire basket is suspended of sufficient weight to hold the specimen sub- 
 merged. (See sketch.) The weight of the basket is balanced with weights 
 added to the other scale pan. The specimen is first weighed in air and 
 then placed in the basket and held completely submerged without touching 
 the water container while the scales are again balanced. If the specimen 
 has a tendency to float, the weight that is added to restore balance is 
 added to the weight of the specimen in air to give the weight of an equal 
 
 -3 
 
volume of water. Conversely, if the specimen has a tendency to sink, 
 the weight that is added to restore balance is subtracted from the weight 
 of the specimen in air to determine the volume. In this case, the weights 
 added to restore balance are added to the same pan as the weighf applied 
 to balance the weight of the wire basket. 
 
 WEIGHTS APPLIED TO 
 REBALANCE SPECIMEN 
 
 WATER LEVEL 
 
 WIRE BASKET 
 
 SPECIMEN 
 
 & 
 
 A 
 
 WEIGHTS APPLIED TO 
 BALANCE WEIGHT 
 OF BASKET 
 
 METHOD B 
 
 Green specimens may usually be immersed for volume determinations 
 in the condition in which they are selected. The determination of volume 
 should be made as quickly as possible after immersion of the specimen, 
 because any absorption of water by the specimens directly affects the ac- 
 curacy of the result. To prevent ovendry specimens from taking up water 
 when they are immersed, they may be dipped in hot paraffin wax. After 
 the paraffin dip, the specimen should be weighed again before immersion 
 and this second weight used in connection with the immersed weight for 
 determining the volume of the specimen. 
 
 A quicker method of treating specimens to prevent water absorption, 
 which is suitable only for softwoods and for hardwoods with small pores, 
 is to dip them in a solution of paraffin wax in carbon tetrachloride (1 
 gram of paraffin in 150 cubic centimeters of carbon tetrachloride). Al- 
 low a few minutes for the carbon tetrachloride to evaporate and then de- 
 termine the volume by one of the immersion methods. The gain in weight 
 due to the thin film of wax is negligible and may be ignored. An advan- 
 tage of this method of treating the specimen is that it may be used satis- 
 
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factorily to determine the volume of air-dried specimens, since the thin 
 film of wax does not appear to affect the subsequent shrinkage of speci- 
 mens when ovendried. 
 
 Rapid Method for Determining Specific Gravity 
 
 The flotation method is a rapid method of determining specific gravity 
 directly, but it is less accurate than the other methods described. A 
 long piece of regular cross section is floated on end in a narrow vessel 
 of water. The submerged portion of the piece, expressed as a decimal 
 fraction of the total length, is numerically equal to the specific gravity 
 of the specimen, based on weight and volume at the moisture condition 
 when tested. 
 
 Determination of Moisture Content 
 
 If the specimen is weighed immediately when obtained as well as after 
 ovendrying, the moisture content may be computed, thus affording both 
 moisture and specific gravity determinations on the same piece. 
 
 ., . / x green weight - ovendry weight x 100 
 
 Moisture content (percent) = — - 2 
 
 ovendry weight 
 
 Weight per Cubic Foot 
 
 The weight per cubic foot of a specimen at the current moisture condition 
 may be determined quite easily from the specific gravity, based on oven- 
 dry weight and volume at test, by the following relationship: 
 
 Weight per cubic foot = Specific gravity x 62.4 x (100 + moisture 
 
 content at test) 
 
 _ 5_ Lculture- Madi son 
 
APR 2 1 19'J up* 1 8 1973 
 
 Table 1. --Specific gravity-strength relations among different species - 
 
 .'«•'» 
 
 Property 
 
 Moisture condition 
 
 Air-dry (12- 
 percent 
 moisture 
 content) 
 
 Static bending: 
 
 Fiber stress at : proportional limit. 
 
 Modulus of rupture 
 
 Work to maximum load. 
 
 Total work 
 
 Modulus of elasticity. . . 
 
 Pounds per square inch 
 
 do 
 
 Inch-pounds per cubic inch. . 
 
 do 
 
 1,000 pounds per square inch 
 
 Impact bending: 
 
 Fiber stress at proportional limit. 
 
 Modulus of elasticity. 
 Height of drop 
 
 Pounds per square inch 
 
 1,000 pounds per square inch. 
 Inches 
 
 Compression parallel to grain: 
 
 Fiber stress at proportional limit. 
 
 Maximum crushing strength 
 
 Modulus of elasticity 
 
 Compression perpendicular to grain: 
 Fiber stress at proportional limit. 
 
 Hardness: 
 
 End 
 
 Pounds per square inch 
 
 do 
 
 1,000 pounds per square inch. 
 
 Pounds per square inch. 
 
 10.200G 
 
 17.600G 1 
 
 35. 6G 1 
 
 103G' 
 
 2.360G 
 
 1 
 23.700G 
 
 2.940G 
 
 1 
 L14G 
 
 5.250G 
 6.730G 
 2.910G 
 
 3.000G' 
 
 1.25 
 
 1.25 
 
 2.25 
 
 Pounds . 
 
 .2.25 
 
 2.25 
 
 25.700G 
 
 32. 4G 1 
 
 72. 7G 2 
 
 2.800G 
 
 31.200G 
 
 3.380G 
 
 94. 6G 
 
 8.750G 
 
 12.200G 
 
 3.380G 
 
 4.630G* 
 
 4,800G <: 
 3.770G 
 
 1.75 
 
 2.25 
 
 2.25 
 
 -The values listed in this table are to be read as equations. For example, modulus of rupturr lor 
 
 green material is 17.600G 125 , where G represents the specific gravity, ovendry, based on voluim- 
 at moisture condition indicated. 
 
 liiiii* 
 
 Z M 23990 F