COMPRESSION FAILURES IN WOOD 

DETECTED BY THE APPLICATION 

OF CARBON TETRACHLORIDE 

TO THE SURFACE 

March 1945 

No. 1591 

UNITED STATES DEPARTMENT OF AGRICULTURE 

FOREST SERVICE 

FOREST PRODUCTS LABORATORY 

M a d i s o n , W i s c o n s i n 

In Cooperation with the University of Wisconsin 
- - - - - - 

COMPRESSION FAILURES IN WOOD DETECTED BY THE APPLICATION 

OF CARBON TETRILCHLORIDE TO THE SURFACE1 

BY 

JOHN P. LIMBACH, Technologist 

Introduction 

Sudden breaking of wood parts in aircraft during service is sometimes due to certain obscure defects termed "compression failures.#2 It is often 

difficult or even impossible to detect with the unaided eye the most minute 

of the compression failures that occasionally occur in wood products 

although even they may be seriously weakening, especially in shock resistance in bending. Any means, therefore, that will render such defects more 

clearly visible is of value to aircraft manufacturers. 

use of Clearing Agents in the Detection 

of Compression Failures 

Wood surfaces showing no indication of compression failures have 

sometimes, upon being varnished, strikingly shown obscure defects that had 

previously been unnoticed. By applying a coat of varnish to a piece of 

wood, the scattering of light from the myriad surface wood fragments has 

been in effect reduced since the varnish, with its smooth surface and index 

of refraction approaching that of wood, immerses these fragments within 

itself to make them invisible. The varnish is a clearing agent and thus 

reveals the subsurface characteristics and irregularities of the wood free 

from the surface bloom caused by the minute wood fragments. Unfortunately, 

by the time varnish is applied, manufacture has approached completion and 

the rejection of defective material at this stage is costly. 

1This report is one of a series of progress reports prepared by the Forest 

Products Laboratory to further the Nation's war effort, Results here 

reported are preliminary and may be revised as additional data become 

available. 

2For further information on the nature, origin, effects, and other methods 

of detection of compression failures see "Detection of Compression 

Failures in Wood" Forest Products Laboratory Report No, 1588, June 1944. 

Report No. 1591 -1-
The surface bloom can, however, be removed long enough for Inspection 

purposes at any stage in manufacturing by the application of a nonswelling 

volatile clearing agent having an index of refraction approaching that of 

wood (about 1.6). Carbon tetrachloride3 meets these requirements well and 

in addition is inexpensive, readily available, and nonflammable. After 

application, it evaporates quickly from a wood surface leaving the surface 

unchanged. Water, in contrast, causes swelling and permanent raising of the 

grain when applied to a wood surface. The freedom from Change after the 

application of carbon tetrachloride is illustrated in figure 1. Carbon 

tetrachloride3 should be used only in a well-ventilated room since prolonged 

exposure to its vapors may be harmful. 

If compression failures are present they can be readily seen on the 

tangential face of a smoothly surfaced specimen after spreading carbon 

tetrochloride over the portion under examination. The minute failures 

become evident to a greater or lesser degree, depending upon the orientation 

of the light and the angle from which they are viewed. Seen directly from 

above, compression failures appear bright when light at a low angle strikes 

directly along the grain and dark when it strikes at right angles to the 

grain, Figure 2 shows a section of Sitka spruce that has compression 

failures, both before and after the application of carbon tetrachloride. 

Compression failures largo enough to be seen without treatment axe 

also rendered more evident by the application of carbon tetrachloride. The 

more advanced failures usually appear dark against the lighter background of 

the wood surface regardless of the orientation of the light (fig. 3). 

The explanation of the somewhat dissimilar effects with minute as 

against gross compression failures is that in the minute failures the only 

function of the clearing agent is to eliminate the surface bloom so that 

the difference in the angle of light reflection caused by fibril displacement is made more evident, while in more severe compression failures the 

appreciable cell-wall dislocation that has taken place permits the clearing 

agent to soak down into the subsurface of the wood to make it appear dark. 

Carbon tetrachloride can be applied to small areas with an eye 

dropper. A brush may be more convenient when applying the reagent to larger surfaces. 

It is believed that the use of such clearing agents as carbon 

tetrachloride should be of considerable value in the detection and elimination, from critical aircraft construction, of that type of wood having low 

strength by reason of incorporated compression failures. 

3 Trichloroethylene has properties that are similar to those of carbon 

tetrachloride. 

Report No. 1591 -2-
Summary 

Carbon tetrachloride spread over a wood surface containing compression failures renders such failures more apparent to the unaided eye. The 

compression failures may be viewed directly from above and the illumination 

provided from the side at a low angle either directly along or perpendicular 

to the grain, Minute compression failures appear brighter than the 

surrounding wood when the light is along the grain and darker than the 

surrounding wood when the light is perpendicular to the grain. Gross 

compression failures appear dark regardless of the orientation of the 

light. 

Report No. 1591 -3-

Figure 2.--A, Section of Sitka spruce with light shining in the direc

orientation. 

Arrow points to minute compression failure which is not visible at 

B, Same section as shown in A with light orientation 

same, but with carbon tetrachloride applied to the surface. Arrow 

compression failure made visible by such treatment. 

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Figure 3.--California red fir board with large compression failure. Ar

point to vicinity of the defect. A, compression failure as seen with

treatment; B, the same area wetted with carbon tetrachloride. 

Z M 60878 F