and or not containing alumina the zinc oxide may be advantageously added in the vitrifiable mixture which will be used to make a reduced glass this oxide permits to avoid the appearance of the amber color due to the formation of iron sulphide which may be produced in this type of glasses thus the zinc oxide may be introduced in the glasses according to the invention at the rate of at least 0 05 when the reported feo fe 2 o 3 total is equal to or greater than around 0 4 in order not to increase the cost of the composition excessively the content of zno does not exceed around 3 by weight the glasses according to the invention may equally contain zirconium oxide this oxide permits a stabilization of the glass and to amelioration of the chemical resistance of the glass particularly its hydrolytic resistance this oxide is introduced preferably in the glasses according to the invention containing few or no alumina in the contents which may attain 1 5 by weight the glasses according to the invention may equally contain cerium oxide in order to increase the absorption of ultra violet radiations the glasses according to the invention.