Forest gllasses of the 16th and 17th centuries were investigated spectroscopicallly (UV, visible, and near IR) and by ESR to determine a relation between the observed colors and the then existing melting techniques. By using model glasses, it was shown that the very wide range of colors found in old gllasses could be obtained from only two elements, iron and manganese, which were found in the raw materials, if the partial pressure was varied during melting. Both of the experimental methods provide information about one side of the iron-manganese redox system. Thus oxygen partial pressures of 10-<s4 to 10 -7>s bar were apparentlly achieved in melting. -- AATA