no 3 2 6h 2 o and 1 0 gram al no 3 3 9h 2 o these minima are equivalent to about 0 095 cr 2 o 3 and about 0 136 each of zno and al 2 o 3 in the impregnating solution in turn they are equivalent on a calculated oxide basis to about 0 024 cr 2 o 3 0 034 zno and 0 034 al 2 o 3 in the consolidated glass for automotive lamp applications it is desirable to provide a peak in the light transmittance curve above 560 nm preferably in the range of 570 580 nm and to provide stable transmittance beyond the peak out to at least 700 nm for an acceptable color intensity with a short impregnation time we prefer to use impregnating solutions containing larger amounts of each salt with an impregnation time of two minutes in particular we employ solutions containing metal nitrate salts in concentrations by weight equivalent to 1 2 cr 2 o 3 1 6 4 zno and 1 6 4 al 2 o 3 these are equivalent to about 0 25 to 0 50 cr 2 o 3 0 4 1 0 zno and 0 4 1 0 al 2 o 3 in the consolidated glass however as shown later the actual amount of nitrate salt and consequent oxide introduced into the porous glass is substantially lower in general impregnating solutions containing the nitrate salts of chromium zinc and aluminum in weight ratios of about 1 1 2 provide optimum glass quality color intensity and hue these ratios may be varied of course the following guidelines indicate the effects that may be encountered when.