In paintings on canvas, there are portions where hardening of oil colors is greatly delayed. This may be a serious hindrance in the production of paintings. The delay always occurs in portions of the canvas where the stretcher is located. The effect of wood used in stretchers on the delay of hardening of oil colors has been investigated. A compression spread method was developed to measure objectively and accurately the degree of hardening of oil color can be measured from the spread-area-ratio. Extractives from western red cedar (Thuja plicata D. Don), were added to the oil colors in small amounts and the degree of hardening was measured. Compared with untreated oil colors, it was found that those with added extractives required required three times longer to harden. It is clear that the delay in hardening is caused by these extractives. When the wood is put in contact with canvas for an extended period, the extractives transfer into the canvas and the delay of hardening of oil colors takes place in the affected regions. -- CAL