The adhesive properties of ion-exchanged Center lignite ash were evaluated using the sticking apparatus. ASTM ash from ion-exchanged Center lignite was suspended on a platinum and heated in a methane-oxygen flame of adiabatic temperature between 1650/sup 0/ and 1750/sup 0/C. The resultant molten slag was dropped onto an oxidized carbon steel surface. The ion-exchanged Center lignite ash had a sticking temperature of 550/sup 0/C. The sticking temperature of the raw Center lignite ash was considerably lower (approx.375/sup 0/C) as reported previously. The higher sticking temperature of the ion-exchanged ash was probably due to removal of sodium and calcium from the Center lignite. The gas-fired vertical tube furnace was used to form deposits with Center lignite, San Miguel lignite, Rosebud sub-bituminous coal and Navajo sub-bituminous coal. The objectives of these tests were to evaluate (1) the effect of oxygen partial pressure during combustion on deposit characteristics of the four coals and (2) the effect of hot zone temperature on deposit characteristics of Rosebud and Navajo sub-bituminous coals. Increasing the furnace oxygen partial pressure from 20 to 60% O/sub 2/ was generally found to decrease the deposit formation rates and strengths. X-ray diffraction analyses of deposits formed during the 60% O/sub 2/ tests indicated that less alkali/silica interactions had occurred compared to the tests performed with 20% excess air. Decreasing the furnace hot zone temperature was found to decrease deposit formation rate and strength for the Rosebud coal. These trends were also observed for Rosebud coal which had been ion-exchanged. 3 refs., 5 figs., 6 tabs.