Clay Firing Pottery
When clay firing pottery and delving into the wonderful world of mud, clay, and pottery, it is quite important to be adept at handling the heat, temperature, kit, and wisdom needed to produce flawless porcelain-like items.
I’m going to walk you through common and not-so-common things you need to think about before, during, and after the clay firing process. As a bonus, we’ll also explore various kilns you can use at home with relatively little fuss. Okay, let’s dive right in.
Before getting into the technicalities, it is quite wise to get up to scratch with the meanings of some terms in order to be on the same page when some processes are applied.
Bloating occurs when you fire clay too fast or irregular clay firing, which means you had variable temperature or you lost power at some point. It also refers to the situation where carbon is trapped within and appears as a bump.
Calcination is a pre-firing technique where most of the air is expelled as well as oxides and residue gases present in the initial clay. Again, some of the cracks in our pottery are there intentionally. There is a unique technique with the exotic name Craquelle where different colors are embedded into cracks after initial firing giving an artistic effect.
And lastly, a bisque is a sort of foreign lingo for a biscuit, which is fired clay that has not been through the glazing process. Okay, glazing, there’s evidence of glazing as early as 600 BC but it is said that the Chinese were doing it even earlier.
Glazing is the dipping or spraying of materials that give a shiny layer to pottery and allow for illustrations and decorative elements on porcelain.
Wedging Before Firing (And why dry is better)
A common problem that occurs within the kiln begins quite before. It involves how well you have done your wedging. Wedging is the process of removing air bubbles from within your clay.
If you don’t wedge there will be spectacular (bad) results in the kiln. As you heat the clay in firing, the trapped bubbles of gas get under pressure and find ways to exit.
This could lead to bloating of the surface, glaze pinholes, and cracks that are difficult to remedy.
Fast firing, when and how to ace this
More drama I’ve suffered when learning how to fire my clay is knowing exactly how fast to fire my items. There are four main stages once the clay hits the oven.
The first involves water, called Water Smoking, so you have to maintain the kiln below boiling point. If you fire too fast or at too high temperatures, it is almost inevitable that the hot steam will explode your items.
It is also wise to make sure your clay items are completely (totally and absolutely) dry before attempting to fire them.
The second stage (dehydration)
The second critical stage is dehydration. Here’s where the chemical composition of the clay changes and the quarts starts to transform.
At this stage, it’s technically not clay anymore. i.e. you cannot crush it, mould it and get the same results. For here again the water changes its molecules and you need a slow cook to get the best results.
If this is not adhered to then the clay is likely to crack and break up.
The third stage is called Oxidation
Here impurities are burned out and your kiln requires lots of Oxygen to get this process seamlessly completed. The vents in the kiln need to be adequate and clear to facilitate this.
Residues like Carbon are removed at this point.
Firing too fast might cause the residues to be trapped within the now almost homogenous clay, leading to a variety of faults including bubbling, cracking, peeling, and bloating of the clay items.
Your finished items might also suffer the presence of a special fungus called mildew. Ugh!
Fourth stage (maturing)
Here your items have shrunk to the recommended levels and the pottery now has adequate strength if all goes well. The clay items are no longer porous and can now be decorated if required.
The surface is smooth like the outer shell of an egg and the color has solidified to give a matte or shiny finish depending on the item or the glazing (if porcelain).
Some Tips When Firing Clay
1. If you’re using an electric kiln to fire clay, make sure you glaze-wash all the shelves prior. This protects your kiln from drips and leaks and makes it more robust as well as last longer.
Clean your kiln often and try as much as possible to remove particles. A vacuum is a good tool to have for this purpose.
Any number of things could cause bad results: it could be a dusty surface, the temperature varying, or different parts of your kiln having different temperatures. It’s best to try different items to get a feel of what works best.
2. It is always better to use cones. Pyrometric cones basically bend due to temperature at different times giving you an accurate reading of the environment within your kiln.
This helps you avoid various problems associated with wrong temperature and fast firing.
3. Choose the best type of firing for what you want to achieve. There are loads of different kilns, tools, ways of firing, from wood kilns, to gas to electric.
These have different outcomes especially with regards to evenness and porosity. For instance, sawdust firing has less porosity than a Raku kiln.
Also, electric kilns are generally unpopular with pros because you can’t do reduction with them. They are popular with newbies because they generally have digital settings and are easy to set.
Types of Kilns
Kilns are manufactured in two kinds; oxidation kilns and reduction kilns.
Oxidation kilns provide an atmosphere rich in oxygen when clay firing while reduction is the very opposite. It is removing the oxygen from the kiln’s atmosphere.
Reduction kilns are gas kilns, wood-fired kilns, pit fire kilns, sawdust kilns, Raku kilns, and sometimes saga kilns.
For those who know, there’s almost nothing more fun than making your own pottery and clay items. Especially when the results come out professional or even buy-worthy.
However, it takes patience and constancy to finally get desirable results. Staying humble and learning from multiple sources will in the end deliver great results and you can see the fruits of your labor all around your house.