Raw Glazing and Single Firing
I talk a lot about single firing, as that is how I fire all of my pottery, but what is it?! In this post, I’ll cover the difference between single firing and bisque firing/ glaze firing, as well as looking briefly at the history of pottery firing. I’ll talk about why I single fire and explain how to raw glaze and program your kiln for single firing.
What is bisque firing, glaze firing and single firing?
Since the industrial revolution, most British pottery has been fired twice. The first firing turns the clay to pottery. This is called bisque firing. The bisque pots are then glazed and fired again, usually slightly hotter, in a glaze firing.
BUT, pottery was not always made like this and you don’t have to do these two firings. My work is nearly all single fired. This means it’s only fired once.
Firings are slow processes. In twice firing, the ‘greenware’ (unfired clay) is bisque fired as follows. Once the clay is dry it is heated in a kiln to somewhere between 900-1000 degrees. This firing allows the clay to become pot and takes in the region of 6-14 hours. A bisque firing is usually done slowly for a couple of reasons. Firstly all the water must have time to escape from the clay. Water is present as both ‘loose water’ i.e. the water in the clay that makes it feel wet, and chemically combined water (the hydroxyl groups attached to minerals in the clay). It all needs to escape. Water trapped in the clay and heated too quickly results in pockets of expanding water vapour (steam) and this results in exploded pottery.
Also, to form into pottery the microscopic structure of the clay essentially has a reshuffle. During these reshuffles (e.g a quartz inversion) the clay expands. If you have uneven heating you get different rates of expansion which can lead to cracking, so a firing will usually go slow through the temperatures that this reshuffling occurs to make sure everything heats up evenly.
The exact temperature and timing of a bisque firing depends on what’s being fired and who has programmed the kiln. Once it’s reached the top temperature the kiln and work is then left to cool down naturally. My modern, well insulated, electric kiln takes around 36 hours to get cool enough to unload.
Once cool, the bisque ware can be removed from the kiln and glazed. It is then loaded back in the kiln and fired again. Glaze firings are often hotter than bisque firings and the kiln is taken to between 1000 and 1300 degrees Celsius, the temperature needed for the glazes to mature. Once again the timings and temperatures depend on what’s being fired, what the clay and glazes are etc. Once again after reaching the top temperature the kiln needs to cool right back down again. More waiting!
In single firing (sometimes called once firing), you essentially skip the bisque firing. You glaze the unfired pots and fire, slowly but just once to turn the clay into glazed pottery.
A short history of single firing
From what I have read, once firing was the standard way of doing pottery here in the UK until the industrial revolution. Indeed, single firing was the standard type firing across the world for thousands years, whether in pit fires or in kilns which were developed around 6000 BC in the middle east . And it wasn’t only for simple, plain work. Think of the decorative ware made in ancient Greece. This was all made through carefully controlled single firings. Once you have kiln technology and can move your pots above the fuel, intricate decoration becomes possible.
Bisque firing appears to have been developed in the 8th or 9th century in the middle east specifically for tin glazes which can crawl if once fired. At this point, lead glazes in that region were still being applied to raw pottery, although it seems like bisque use was also spreading.
It’s quite hard to find references online but it seems like it wasn’t until much later, somewhere between the 17th and 19th centuries, that twice firing became common place in England. This coincides with the advent of massive industrial potteries like those in Stoke on Trent so it may have been about reducing the fragility of the stacks of pots waiting for decoration or about saving money on skilled workers needed for intricate raw glazing. Tin glazes and majolica ware were also popular in England around this time so perhaps the bisque firing came with these glazing techniques (If you have any good references on this I’d love to hear from you).
The amount of fuel the industrial bottle kilns use is immense so it’s amazing to me that they wanted to add firings! However, for the potteries of Stoke on Trent wasting energy didn’t seem to be much of a concern. Apparently 90% of the heat generated in these kilns was wasted! The age of industry, when we burned with abandon (has it stopped?!)!
Whatever the reasons, potters largely stopped single firing for at least a hundred years but since the 1970s it seems to be coming back to studio potteries. Tristam Fran, Andrew Holden, Steven Hill have all written on the subject. More recently Simon Leach has made several YouTube videos covering his experiments.
Why single fire?
There are two main reason; time and energy.
Single firing basically skips the bisque firing step, so is much quicker.
As an impatient potter, single firing is a great complement to my short attention span. I can throw pots on a Monday and have them finished and out of the kiln by the weekend (which in the world of pottery is super speedy). It also adds less stops and starts which I find helps keep my process fluid!
Single firing saves energy
The really big one for me is that single firing massively reduces energy consumption. Firing is the biggest outlay of energy in a ceramic studio, so any reduction in this is really important. Of course this makes it cheaper, which every business wants, but also, single firing can massively reduce a potter’s carbon footprint.
How much energy do I save single firing.
One single firing of my electric 80L kiln to 1210oC uses around 35kWH of energy. A glaze firing using around the same amount. A bisque firing to around 1000 would use approximately 20 kWh electricity (this one is an estimated figure based on other peoples firing data)
So twice firing would use 35 + 20= 55 kWh
While single firing uses just 35 kWh
Also just for context, my kiln fully loaded holds about 25-30 mugs.
This obviously saves money on my electricity bill but we can look in terms of Carbon footprint too.
Looking online you can get a picture of how much carbon a kWh of UK electricity ‘costs’. In 2023 the UK’s yearly average figure was 162g CO2/kWh. So, single firing in my kiln has a carbon output of :
35 * 162 /1000 = 5.67 kgCO2
while twice firing has a carbon output of :
55*162/1000 = 8.91 kgCO2
So, by single firing I’m saving 20 kWh or 3.24 kgCO2 for every kiln load of pots.
In 2025 the climate crisis is firmly on our conscience, and more and more potters are trying out single firing. Will you join us?!
How to single fire
If you want to move from twice firing to single firing, two main things will need to change in your working process. First you need to raw glaze, which is a slightly different technique than glazing bisque ware. Secondly you will need to program a new firing schedule for your kiln.
It may also be that you need to fiddle about with your glaze recipes and what clays you use, but don’t worry about that to start with, just do some tests using your existing materials, odds are you’ll get something working. I highly recommend raw glazing one or two test pots before doing a whole batch. You’ll want to test each glaze and clay combination.
How to raw glaze
The raw glazing process can be a bit finickety. When you are dip glazing a bisqued pot you can essentially just dunk it in to the liquid glaze. It is pot so will not go back to clay when it gets wet. The unfired greenware however, is much more fragile. It needs to be handled gently. As you may imagine, the greenware softens and expands when coated in the liquid glaze, so slow and careful wins the race.
Many people recommend spray glazing for single firing, I haven’t tried that simply because I haven’t got a spray booth set up. In my experience brushing glazes work fine but in this article I’m going to talk about dipping glazes because that is what I primarily work with. You can easily adapt this advice for brushing or spraying.
You can either raw glaze when the pots are bone dry or when they are leather hard. I have found what’s best varies between clays. In my experience some clays will crack or ‘delaminate’ when they glazed are bone dry. Delamination is a term I found in a Steven Hill article and it is when the layers of clay literally pull apart and you get bubbles within your pot walls. Overall I have had more success raw glazing at leather hard.
Here’s my method.
When my pots are leather hard, I wax the bases. I have a damp sponge and a loop tool to hand to tidy up any mess.
Once the wax is dry (5-15 mins) I hold the pot in my hands, not with tongs – points of pressure are much more likely to lead to breakages.
I first glaze the inside, I pour glaze into the pot and immediately swill round and pour out. Don’t squeeze it! Straight away I wipe any glaze off the rim and remove any drips I don’t want with a damp sponge, bearing in mind that pressing hard will indent the clay. I then put the pot down upside down (so it doesnt pool) and wait for this layer of glaze to dry. This takes longer than when glazing bisque pots as you are waiting for the clay to dry out again too. On a warm day it can take 10 minutes, if it’s a cold damp day I sometimes wait a few hours.
Once it’s about as dry as it was to start with, I dip my pot once into the glaze the right way up. For my glazes it’s just a few second dip but this depends on your glaze thickness. If your glazes are ones you use for bisque ware, you might want a very slightly longer dip that you’d usually do. As a rough guide, I usually aim for just under a credit card thickness of dipping glaze on the pot. Once again let it dry.
Lastly, I dunk the rims. Any runs can be gently removed with the loop tool. And any pinholes rubbed over with the fingers once the layer has dried.
I’ve found the drying between stages is really key. If I want to add overlapping glaze layers, I wait for the lower layer to be dry before adding another or the glaze can flake off or even worse the clay instantly delaminates. Extra layers can be dipped but I often brush.
After glazing, you then want to dry your pot out as you would before a biscuit firing.
Firing Schedule
Once your pots are dry they can be fired. I admit, when I’m on a deadline, I sometimes load mine straight into the kiln and then do a slow preheat but it’s meant to wear your elements quickly and use more energy so and is definitely not best practice! Also, my work is light and thin. If you are doing sculpture or make heavy or thick ware then let it dry.
Leave it to dry as you would for a bisque firing. As a rule of thumb, it wants to be room temperature to the touch, not cold. Moisture, not air bubbles are what lead to explosions.
Try not to stress about breakages. It’s a common fear for people starting out in single firing but once you’ve glazed your pieces, in my experience, single firing does not lead to more explosions or cracks during firing than a bisque firing does. You’ll get the same damage due to moisture and any cracks that form would have also formed in bisque firing. So dry and slow is key.
A single glaze firing schedule is at its most basic level a whole biscuit cycle that then goes straight into the last bit of a glaze cycle. If you have a biscuit and a glaze cycle that work well, I suggest you just add them together.
Here’s my current single fire stoneware cycle as a starting point. Its a cone 6-7 firing. Bear in mind I’m going really slow at the start to match my tendency for firing slightly damp (but thin pots).
- 0-100oC @ 30oC/hr
- Hold for 1-3 hrs at 100oC (these first two slow stages are for removing moisture. I do 1 hr in summer here, 2 hours in winter when everything is damp)
- 100-600oC @ 70oC/hr (slow to allow chemically combined water to escape and and slow through quartz inversion when you can get cracking)
- 600-950oC @150oC/hr
- 1 hr at 950oC (this is my top of bisque soak, I cut this time to 20 mins when I’m using white clays)
- 950-1210oC @200oC/hr (my kiln doesn’t achieve this rate, but I’m essentially telling it to go as fast as it can. You can aim for whatever temp your clay/glazes need here)
- 20 min hold at 1210oC (top temp soak).
And then you wait as normal for it to cool, open it up and wonder at the marvels of science and art!
So, are you going to try single firing? Or do you already single fire and think I missed something key?
Do drop me a comment here, or a message on instagram if you have any questions. There’s also a really great single firing community on facebook that I really recommend joining for loads of samples firing schedules and general support on your single firing journey.
You can browse my single fired pottery here
Useful links and resources
Steven Hill article: https://static1.squarespace.com/static/555a4afbe4b06f6e6f42474f/t/55712d4be4b072dc472942ce/1433480523001/An+Approach+to+Single+Firing-Further+In.pdf
Ceramic Review Issue 46 Jul / Aug 1977 Contents… : Single Fire Glazing – Dennis Parks: https://ocean.exacteditions.com/issues/66156/page/4?rc=cbded4bf-d38e-4932-96a0-97694481a793
Other interesting reading:
What happens during firings: https://ceramicartsnetwork.org/daily/article/Firing-Clay-The-Lowdown-on-the-Ceramic-Firing-Process and https://digitalfire.com/article/firing%3A+what+happens+to+ceramic+ware+in+a+firing+kiln
Bottle kilns: https://re-form.org/middleportpottery/about-middleport-pottery
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