Through the years, Contreras has experimented on non-traditional art materials in his quest for contemporary Filipino art. Aside from the travieza, he explored using logging refuse (roots and branches) in order to pave way for reforestation efforts and to help organize backyard industry to onion farmers who supplied and eventually were trained to use the hardwood refuse salvaged from the river as art material. After the devastating Mount Pinatubo eruption, Contreras experimented on using the volcanic rocks and lahar to sculpt human figures reminiscent of the indigenous Aeta and lowland communities who were displaced by the volcanic eruption; these stone sculptures were exhibited at the Cultural Center of the Philippines, paving way for further exploration of the volcanic materials as art media.