Callum graduated with a BSc in chemistry from the University of Bristol and then worked as a technician in the Bristol Royal Infirmary. He later joined the University of Bath School of Materials to research the degradation of polymeric materials under the supervision of Dr. Martin Ansell. Callum then moved to what was then called Bristol Polytechnic to study the interaction of toxic gases with organometallic compounds for use as gas sensors, work for which he was awarded a PhD. Following this he then went to Bristol University to study the stabilisation of non-aqueous colloidal dispersions. He then went to the School of Chemistry at Bangor University to investigate the use of organometallic materials for non-linear optical devices. In 1994 he took up a lectureship in Wood Science at the then School of Agricultural and Forest Sciences, Bangor University. In 2007 he moved to the Centre for Timber Engineering to take up the Edinburgh Research Partnership Chair of Materials Science, a position he held until 2013. He is now running his own consultancy company (JCH Industrial Ecology Ltd), a company specialising in researching the use and environmental impacts of materials use, concentrating especially on renewable materials.