A design student from University of Wisconsin-Milwaukee decided to build a 3D printer that would use clay as the printing material for a fabrication class. His delta style 3D printer is still a work in progress, however it does work and is capable of printing objects as tall as 3.5 feet. The clay printer was based off of the open source Rostock design, however he scaled it up and modified it to print using his own clay extruder. You can find out more about the clay 3D printer over in the following article here: http://3dprint.com/69789/clay-delta-3d-printer/
Below is a picture of the machine in action as well as a piece which has been printed with Martin's machine: