Simply put, annealing is the process of slow cooling glass from its molten state to its solid state. The time it takes to anneal glass will differ based on objects of different thicknesses, compositions, and detail. Here, I'm going to focus on soda-lime glass with a COE (coefficient of expansion) of 96, because that's what I am familiar with.
Above, you see inside a glass furnace, held at a temperature of around 2100°F to keep the glass at its molten state. This is where handblown glass begins. On the end of a glass rod, you will spool or 'gather' glass out of the furnace in a constant turning motion, so as to keep the glass on center with the rod. Here, the glass has the consistency like that of honey, so it will readily drip to the floor if you are not constantly turning.
Photo by Johannes W on Unsplash
After the glass is gathered and exposed to the ambient temperature, the glass will start to harden. The bright orange glow of the glass will dissipate, indicating that your honey-like glass is more like hard candy with a gooey center. If left to cool, it will become completely clear and entirely hard. Several minutes later, the glass will crack or explode off of the metal rod that it was gathered on because the glass just went through a process of improper annealing. The rate at which the glass cooled was too rapid, leading to internal strain, causing it to explode.
To prevent strain from forming in the glass, glassblowers will place their finished pieces in a kiln, or annealer for slow cooling.
I'm no scientist, so I'll keep my annealing explanations simple and non-technical. Exact annealing temperatures and strain points vary based on glass brand and formulations.
There are three key factors to keep in mind:
Annealing temperature
There can be slight variations in recommended annealing temperature based on the brand of soda-lime glass used, but it is typically at around 900°F. The annealing temperature is the temperature at which glass is physically solid, but at a molecular level, hasn't formed its proper amorphous structure.
Annealing hold
This is the time held at annealing temperature. The hold can vary in length from none to hours depending on the complexity of the piece. The purpose of this step is to insure that the glass is uniformly heated at annealing temperature prior to cycling through the annealing schedule.
Strain point
The temperature at which glass has finished forming its molecular structure and rigidity. The strain point is typically at around 700°F.
For example, I recently annealed my Blahbs and Squiggles cups with a similar schedule below. This is with a small, Paragon front loading kiln, hooked up to a Digitry.
Step 1: 30 min - 900
Step 2: 2 hours - 700
Step 3: 6 hours - 100
OR
Step 1: 900 - hold 30 min
Step 2: 133 deg/hr - 700 - hold 0 min
Step 3: 150 deg/hr - 100 - hold 0 min
Given the thickness and shape of the cups, this leans towards a conservative program. As a general rule, if you can afford to wait, you don't want to rush an annealing cycle.
If for whatever reason, I need to cut time in this program, I may decrease the hold time on step 1, and cut down the time on Step 3, with adjustments on step 2 to make the program look like this:
Step 1: 10 min - 900
Step 2: 1 hour, 40 min - 700
Step 3: 3 hours, 40 min - 400
There are a number of factors to consider when customizing an annealing schedule. These are some things to consider:
How complex is the work? Do you have thin parts and thick parts in one piece? Are there many assembled pieces in the work? You may consider increasing the hold time.
Are there a lot of different colors involved? You may need a longer anneal to strain point.
Is the kiln very insulated? Sometimes the rate at which you program is faster than the kiln will allow.
Is the kiln not well insulated? You may we wary of cooling too quickly, especially if you decide to idle from a high temperature.
How packed is the kiln? Glass is very insulating. A densely filled kiln will cool much slower than one that is sparse.
How big is the kiln? It takes a lot more energy to cool a large kiln than it does to cool a small kiln.
Are there different shaped items going into the kiln? To be safe, default to the thickest glass piece being annealed.
Annealing glass can be explained scientifically, but there is wiggle room to customize your program if you know what you're doing. Being able to assess your own unique circumstances in glass making is an important skill to have in being well rounded, and in understanding the material.
Lastly, if you're wondering when you can open the kiln, it depends. For straight forward blown work, I am comfortable with cracking the kiln around 150 degrees, and once it hits a temperature where I think to myself, " I think Texas or Arizona gets this hot in the summer" I will open the kiln door. :)
Happy annealing!
For reference, Bullseye glass has a thick slabs annealing chart you can check out. Bullseye glass does not have a COE of 96, but I think this still provides a good reference for understanding annealing rates.
If you have some $$ to spend, you may look into buying a polariscope. It can be used to identify strain in glass.