The term "glass slag" denotes a special kind of glass. It's a type of glass that was very popular during the 1890's. At the time, blacksmiths knew about this glass-like byproduct when processing iron ore. This glass-like product was known as slag. Hence, the name glass slag, which also goes by other names such as malachite glass and marble glass.
The easy way of identifying slag glass is by recognizing the colors. It usually comes with creamy or white color with a swirling of a different color.
Another thing to keep in mind is that the more white or cream color it has, the more it will seem older. Also, the swirl colors will indicate different eras. For example, dark brown, green, blue and purple are the earliest, while orange and red are more recent. It's a thing to remember if you want to pick a slag glass lamp with the goal of portraying a certain era.