An optical system has been developed to characterize transparent organic alloys during their directional solidification in situ and in real time inside bulk samples with a high aspect ratio. Standard solidification experiments are performed within thin glass slides where solidification behaviour is modified compared with bulk solidification due to the space constraint. On the other hand, the interface can be easily observed via a microscope. For bulk samples, a more complex system had to be implemented. We designed a specific glass observation cell and an adapted solidification furnace. Optical elements were integrated inside the glass observation cell containing the solidifying alloy. The resulting glass observation cell can be easily used as a tool for many optical characterization methods. Here we use the system to generate live images of the solid-liquid interface. These images, recorded on video, provided very interesting and fruitful information on the dynamic phenomena appearing at the interface. The whole interface as well as specific details of the interface could be observed. The images showed a resolution of a few micrometres, suitable to characterize interface features, as well as a high contrast and a constant magnification. As a consequence, further image processing to quantitatively characterize the solid-liquid interface could be easily performed. A method to determine the average radius of curvature of a cellular array was also implemented and is presented here.