Glaze firing is often the most exciting, but nerve-wracking part of making pottery.
After all, this is the point where weeks of work finally reveal themselves. The dull, chalky-looking glaze you've carefully applied transforms into the finished surface of the piece.
It's also the point where things can go the most wrong.
Even experienced potters occasionally open their kiln and find a result they weren't expecting. Glazes are influenced by a surprising number of factors, including firing temperature, cooling speed, glaze thickness, clay body, and even where a piece sits inside the kiln.
The good news is that glaze firing isn't magic.
Once you understand the basics, keep good firing records, and gain some experience, glaze firing becomes much more predictable than many beginners expect.
In this guide, we'll walk through the basics of glaze firing, explain why glazes behave differently, and help you avoid some of the most common mistakes beginners make.
What Is Glaze Firing?
Ceramic pieces typically undergo two firings:
2. Glaze firing
Glaze firing is the second firing. During this stage, a glaze is applied to bisque-fired pottery and then fired in the kiln until the glaze melts and bonds to the surface of the clay.
This process creates the finished surface of the piece. Depending on the glaze used, the final result may be glossy, matte, textured, colourful, decorative, waterproof, or food-safe.
Many glazes look completely different after firing than they do when first applied. A bucket of dull grey glaze can sometimes produce a vibrant blue, green, or red finish after firing.
Once the glaze firing is complete, the pottery is ready to use or display.
Glaze Firing Basics
Before we go on, let's clear up some common glaze firing terminology.
Glaze Firing – The second firing of a ceramic piece, where glaze is melted and fused to the clay body.
Vitrification – The process of materials becoming glass-like when heated to high temperatures.
Cone – A measure of heatwork used in ceramics. Cones account for both temperature and time.
Heatwork – The combined effect of temperature and time during a firing.
Oxidation – A firing atmosphere with plenty of oxygen, commonly found in electric kilns.
Reduction – A firing atmosphere with limited oxygen, commonly used in gas and wood-fired kilns.
Soak/Hold Time – A period where the kiln remains at its final temperature before cooling.
Witness Cones – Pyrometric cones placed inside the kiln to verify that the correct heatwork has been achieved.
Why Do You Need To Glaze Fire?
Glaze firing is an essential part of the ceramics process. It transforms a bisque-fired piece into a finished ceramic object.
1. Makes Pottery Waterproof
Most clay bodies remain porous after bisque firing.
Glaze firing melts the glaze into a glass-like coating that helps make pottery watertight and suitable for everyday use.
2. Adds Colour And Decoration
Glaze firing brings colour, texture, and surface effects to pottery.
During the firing process, the glaze melts and develops its final appearance.
This transformation is one of the reasons so many people become fascinated by ceramics. The final result can be dramatically different from what the piece looked like before it entered the kiln.
3. Improves Durability
The fired glaze creates a protective surface that helps resist stains, moisture, and wear.
This makes pottery more durable and easier to clean.
4. Creates Different Surface Effects
Different glazes and firing temperatures can create a huge range of effects, including:
-Glossy finishes
-Matte finishes
-Satin finishes
-Crystalline surfaces
-Speckled effects
-Reactive colours
How To Successfully Glaze Fire
Here are the steps you should follow to give yourself the best chance of a successful glaze firing.
1. Prepare Your Bisque Ware
Before glazing, make sure your pottery has been successfully bisque fired.
Inspect each piece for cracks, chips, or defects that may worsen during firing.
Small cracks rarely improve in the kiln and often become much more noticeable after glaze firing.
Use a soft brush to remove dust from the surface. Dust can interfere with glaze adhesion and may lead to firing defects.
2. Apply Your Glaze
Glazes are available in several forms, including brushing glazes, dipping glazes, and powdered glazes that are mixed with water.
Always stir glaze thoroughly before use.
Apply glaze evenly across the surface of the pottery. Uneven application can lead to defects such as crawling, pinholes, or inconsistent colour.
Allow the glaze to dry completely before loading the kiln.
If you're disappointed with a glaze result, application thickness is often one of the first things worth checking. Even a good glaze can behave poorly if it's applied too thickly or too thinly.
3. Clean The Bottoms Of Your Pots
This is one of the most important steps in glaze firing.
Any glaze left on the base of a pot can melt and permanently stick the piece to the kiln shelf.
Most potters only need to make this mistake once before becoming very careful about cleaning foot rings.
After glazing, wipe the foot ring and lower edge of each piece clean.
Many potters also use wax resist to help keep glaze away from the base.
4. Load The Kiln
Unlike bisque firing, glazed pieces should never touch each other.
During glaze firing, the glaze becomes molten. If two pieces touch, they can fuse together permanently.
Leave space between all items and ensure there is sufficient clearance around shelves and kiln walls.
If you are firing glazes that may run, consider using kiln wash on your shelves or placing catchers underneath the pieces.
This is where having a large kiln can come in handy. A little extra space can save a lot of frustration later.
5. Fire The Kiln
Set your firing schedule according to the glaze manufacturer's recommendations.
Different glazes mature at different temperatures, so always check the firing range before starting.
Many potters also place witness cones inside the kiln to confirm that the desired heatwork has been achieved.
While modern controllers are incredibly accurate, witness cones remain one of the best ways to verify what actually happened inside the kiln.
6. Allow The Kiln To Cool
Once the firing is complete, allow the kiln to cool naturally.
Opening the kiln too early can cause thermal shock, resulting in cracked pottery or damaged glazes.
The temptation to take a quick look is always strongest after an exciting firing, but patience is usually rewarded.
As a general rule, wait until the kiln has cooled below 200°C before opening the lid or door.
7. Hold your breath, wince and open the lid
Hopefully everything should look great. If not - it's all part of the learning process!
Glaze Firing Temperatures
The correct glaze firing temperature depends on the glaze and clay body you are using.
Low Fire Glaze Firing
Low-fire glazes typically mature between cone 06 and cone 04, approximately 950°C to 1060°C.
These glazes often produce bright colours and decorative effects.
Mid Fire Glaze Firing
Mid-fire glazes typically mature around cone 5 to cone 6, approximately 1180°C to 1230°C.
This is one of the most popular firing ranges for electric kilns and functional pottery.
High Fire Glaze Firing
High-fire glazes generally mature between cone 9 and cone 11, approximately 1280°C to 1310°C.
These firings are commonly used for stoneware and porcelain and can produce highly durable ceramic surfaces.
Example Glaze Firing Schedule
A typical glaze firing schedule for an electric kiln may look something like this:
-Ramp at 150°C per hour
-Fire to 1125°C
-Hold for 30 minutes
-Allow the kiln to cool naturally
The exact schedule will depend on the glaze, clay body, kiln, and desired results.
Always follow the manufacturer's recommendations where available.
Why Do Glazes Behave Differently?
One of the things that surprises many beginners is that the same glaze can produce very different results depending on how it is fired.
Several factors can affect the final appearance of a glaze.
Firing Temperature
Many glazes are designed for a specific firing range.
If a glaze is fired below its recommended temperature, it may appear dry, rough, or underfired.
If fired too hot, the glaze may become excessively fluid, lose detail, or run down the pot.
Cooling Speed
The cooling cycle can have a significant effect on the final result.
Fast cooling often produces brighter, glossier surfaces, while slower cooling can encourage crystal growth and create more matte or textured finishes.
Many potters spend years experimenting with cooling schedules because the effect can be surprisingly dramatic.
Kiln Atmosphere
In oxidation firings, which are typical in electric kilns, glazes generally produce predictable and consistent colours.
In reduction firings, which are more common in gas kilns, the lack of oxygen can dramatically alter glaze colours and surface effects.
For example, copper-based glazes may appear green in oxidation but develop rich red tones in reduction.
Glaze Thickness
The thickness of glaze application can also affect the final appearance.
A glaze applied too thinly may appear patchy or dull, while an overly thick application can result in running, crawling, or unexpected colour changes.
If you've ever wondered why your piece doesn't look like the example on the glaze jar, glaze thickness is often one of the reasons.
Clay Body
The same glaze can look very different on different clay bodies.
A white porcelain clay may produce brighter colours, while a darker stoneware body can mute colours and influence the overall appearance of the glaze.
How Can You Predict How A Glaze Will React?
One of the challenges of glaze firing is that the same glaze can produce different results depending on how it is applied and fired.
While there is always some experimentation involved, there are several ways to improve your chances of getting predictable results.
Read The Manufacturer's Information
Most commercial glazes include a recommended firing range, such as cone 6 or cone 10.
This information is usually the best starting point.
Use Test Tiles
Many experienced potters create test tiles before glazing finished pieces.
A simple test tile allows you to see colour, surface finish, movement, and compatibility with your clay body.
It can feel like extra work at first, but test tiles are far less frustrating than discovering a problem after firing an entire kiln load.
Look At Examples Fired On The Same Clay Body
The same glaze can look very different on porcelain, white stoneware, or darker stoneware bodies.
Whenever possible, look for examples fired on a clay body similar to your own.
Understand Whether A Glaze Is Stable Or Reactive
Some glazes are designed to produce consistent results.
Others are intentionally reactive and may vary depending on firing temperature, cooling speed, thickness of application, and position within the kiln.
If a glaze is marketed as reactive, expect some variation from firing to firing.
Keep A Firing Log
One of the best ways to improve consistency is to record:
-Clay body used
-Glaze used
-Number of coats
-Firing schedule
-Peak temperature
-Hold time
-Results
Many potters can remember a great result but struggle to remember exactly how they achieved it six months later.
A firing log quickly becomes one of the most valuable tools in your studio.
You can download a kiln firing log for free - here
Test Before Committing
Even experienced potters test new glaze combinations before using them on important work.
A small test firing is usually much cheaper than discovering a problem after a full kiln load.
Common Glaze Firing Problems
You can read more about common glaze firing problems and how to avoid them here.
Pinholes
Pinholes appear as small holes in the glaze surface.
They are often caused by gases escaping from the clay or glaze during firing.
A slower firing schedule or additional hold time can sometimes help reduce pinholing.
Crazing
Crazing appears as a network of fine cracks in the glaze.
This usually occurs when the glaze and clay body expand and contract at different rates.
The long-term solution is to use a glaze that is better suited to the clay body.
Crawling
Crawling occurs when sections of glaze pull away from the clay during firing.
Common causes include dusty bisque ware, overly thick glaze application, or poor glaze adhesion.
Glaze Running
Some glazes become very fluid during firing and can run down the side of the pot.
Applying glaze too thickly can increase the risk of running.
Always test unfamiliar glazes before using them on important pieces.
Cloudy Clear Glaze
A clear glaze may appear cloudy if it is not compatible with the clay body or firing schedule.
Testing different glaze and clay combinations can often solve the issue.
Glaze Firing Pottery Tips
-
Always use witness cones to verify the actual heatwork inside your kiln.
-
Match your glaze to your clay body and firing temperature.
-
Keep a firing log so you can track successful schedules and troubleshoot problems.
-
Test new glazes on test tiles before applying them to finished work.
-
Ensure all glaze is removed from the bottom of your pots before firing.
-
Be patient when cooling your kiln. Opening too early is one of the most common causes of cracking.
One of the things that makes glaze firing so rewarding is that there is always something new to learn. The more notes you keep and the more you test, the easier it becomes to understand how your glazes, clay bodies, and kiln behave together.
Frequently Asked Questions
What temperature should I glaze fire pottery?
The correct temperature depends on the glaze and clay body. Common glaze firing temperatures range from around 950°C for low-fire earthenware to over 1280°C for high-fire stoneware and porcelain.
Can glazed pots touch in the kiln?
No. Glazed surfaces can melt during firing and fuse together permanently if they come into contact.
How long does a glaze firing take?
Most glaze firings take between 8 and 12 hours, although the exact time depends on the kiln size, firing schedule, and target temperature.
Can I glaze fire without bisque firing?
Yes. This process is known as once-firing or raw glazing. However, it generally requires more experience and careful control than the traditional bisque firing and glaze firing process.
When can I open my kiln after glaze firing?
It is usually recommended to wait until the kiln has cooled below 200°C before opening it. Opening too early can lead to thermal shock and damaged pottery.
Related Articles:
-How Much Does a Kiln Cost to Run?