A fragment of 17th century ruby red Kunckel glass has been investigated to elucidate its internal structure. It is known that gold nanoparticles can cause a red colour. It will be demonstrated that the gold particles found in this piece display a cubo-octahedral morphology and have the right sizes to provoke the proper colour.Furthermore, iron containing particles were abundantly found in the glass matrix. The larger ones proved to be -Fe2O3 particles and it is suggested that they too have an influence on the colour. The composition and structure of the smaller particles was investigated using HRTEM and EELS and most of them are also -Fe2O3, although some are characterised as Fe3O4. These Fe3O4 particles may have caused an internal reducing environment that facilitated the reduction of gold ions into gold particles.