The collapse of the Roman empire led to the creation of a wide variety of glass styles like forest glass. The Roman empire had been making glass for hundreds of years so they had lots of experience making glass they also had natron but the glass they made was clear and thin. 

After the collapse of the Roman empire people closer to Rome  did not have to worry about making glass because it was still being produced. But people farther away from the Roman empire was worrying more about framing and staying alive than making glass. 

 The demand for glass increased people father away from Rome started experimenting new styles of glass They used the materials they had on hand to make forest glass since they did not have natron.