Spiess has worked with many types of materials, including wood, cloth, steel sheet, plastic, and rubber; he has also experimented with others such as copper, tungsten and, more recently, acrylic.
For Spiess “malleability” is fundamental, the possibility of going from the “rigid” to the “soft”, a phenomenon that inspires him to “create worlds”. Currently, he favors the concepts of “light”, “brightness”, “reflection”, “luster” and “transparency”; he prefers materials that he judges as “repellent”, hard and compact, from which he exploits their vulnerability to fire. Regarding acrylic, he expresses that he is surprised by how a sheet can be modified to turn it into a sphere, by means of air and fire, and points out that it has a special relationship with the heat of the material, since it generates in him an “intangible chronometer” that indicates the perfect time to transform it. According to him, this material “projects the brilliance that life radiates, evoking a mirror where we can see ourselves.”
Spiess defines himself as one of the few artists who works with acrylic in the way he does, because he addresses his own creative processes, different from those implemented in the industry.