The collapse of the Roman Empire led to the creation of a wide variety of glass. It caused trade and knowledge to spread throughout Europe. Roman glass was high-quality but skills got worse in Northern Europe. When the Roman Empire collapsed, glass making slowly improved. It brought out more creativity to explore new ways for glass making, such as using new ingredients. The Romans recycled their old canton recipe and experimented with different sources of ash. They discovered new colored glass thousands of years later. They also discovered additives, such as salt elements, that produced different colors. Stained glass windows increased demand for colored glass. They installed new colored stained glass windows in churches. Roman glass making skills increased all over the Middle Ages.