The Romans Loved to make glass. They did it on a regular basis, but when the Roman Empire started to collapse people moved out of Rome. The people who had moved out of Rome still wanted to make glass. Sadly they did not have the tools.
When the Roman Empire collapsed, the making of glass fell out of fashion. It was not completely forgotten, but it was very unpopular. The Romans still wanted glass. Some of the Romans turned to other sources for glass while northern Europe started experimenting. They were trying to make glass with different ingredients and one of the ingredients they used was plant ash which turned the glass dark brown or green. The reason people were trying to make forest glass was because they did not have the skills or the tools to make regular glass.
To recap, The Romans liked to make glass but when the Roman Empire collapsed the people moved out. They did not have the tools to make regular glass so they started experimenting and used plant ash which created a thick dark greenish-brown colored glass. With all of the experimenting a lot of different glass types were invented which created a wide variety of glass styles.