Understanding the compressive behaviour of mortarless joints is necessary to optimise refractory masonry structures. This study investigates joint closure using Digital Image Correlation, a contactless measuring method. Results show that the gap thickness of the dry joint is mainly due to the nonplanarity of contacting surfaces, the surface roughness playing a secondary role. The process of joint closure involves roughness crushing and the adjustment of surfaces, and is strongly heterogeneous, orthotropic and nonlinear. Local joint opening is observed during the first stage of the global joint closure process. This phenomenon is due to rigid body motion which induces a plane rotation of the upper contacting bricks.