The LOCATION@1 learned that a gasoline gel increased the range and effectiveness of flamethrowers , but its manufacture was difficult because it used natural rubber -LRB- latex -RRB- , which then was expensive and in high demand for other war effort purposes -LRB- e.g. truck tires -RRB- .

Gasoline gel was difficult to manufacture because it used natural rubber , which was in high demand and expensive .
