Then, another problem was discovered. Many of the glass colors did not stand up to year-in, year-out exposure to the direct rays of the sun. The result was a stained glass scene of deteriorating beauty. Some colors darkened or changed over time, while others faded away.

Red, a very important color for use in a stained glass window, was especially vulnerable to fading. Artists in many countries worked to produce a red glass that would hold its color through the years under the direct sunlight that passed through the windows. Eventually a permanent red color was developed by adding small amounts of gold to the glass. This significantly increased the cost of the glass, but the red color was achieved. Even today, if you purchase a red sheet of glass it will cost significantly more than any other color.