Glazing, also known as 3rd stage creosote, is basically unburned carbon that is very dangerous. If it gets too hot it can combust and become the catalyst for a huge fire.

This level of creosote can be caused by burning too wet of wood or wood having a moisture content greater than 10-15%. If the wood is too wet, the fire has to use more of it’s energy to dry the wood before it starts to pyrolize or break down before combustion. When more energy is used the efficiency of the fire decreases or begins to burn cooler. Cooler gases don’t rise as fast and they stay in the chimney longer. When it has a longer “Residence time” creosote is formed.

Improper grates can also lead to Glazing. Many fireplaces come from the manufacturer or are left by the builder with steel bar grates. Steel bar grates are not condusive to an efficient fire. As the wood breaks down, large pieces can fall to the firebox floor and begin to cool off thus lowering the efficiency. Cast Iron grates are recommended due to there ability to retain heat and radiate it into the room because of their mass. Also, because the gaps in the grate are much smaller and the wood must basically burn to ash before it can fall.

Then, when you add more wood, it will break down and burn faster.