If you are wondering what pit-firing pottery is, then in simple words, it is the authentic method of baking clay. It is an atmospheric process done typically in a pit (a hole in the ground), where clay pots are burnt to derive various patterns and designs. However, while doing pit firing, one must always remember to use low-fire clay with grog to help the pottery piece withstand thermal shock.
You can also use high-temperature clays for the process, but then you need to bisque it initially before placing it in the pit firing set-up. Many potters prefer this technique to prevent damage like cracking and shattering of the pots. Besides, clays dug directly from the Earth tend to become more suitable and profitable after adding volcanic ash or grog to them. It also helps to resist severe temperature differences and serves as be good element for pit firing.
So, now in this article, we will discuss the top 4 clays that can be used in pit firing.
What are the Best Clays to Use for Pit Firing?
The following are some of the best clays to be used in the pit firing process. Let’s have a glance:
1. Low-Fire Earthenware:
Earthenware clays preferably the low-fire ones tend to be more porous and cause low shrinkage, which makes them ideal for planters. This clay is non-vitreous and opaque, so after being fired, it becomes softer and opens up gradually. And this makes the low-fire earthenware clay to be one of the best for the pit firing process.
Due to its high porosity, low-fire earthenware clay has a maximum water absorption range of 5-8%. And that is why it needs to be glazed to become watertight. Though this particular clay has lower mechanical strength, its porous nature makes it capable of withstanding thermal shock much better. Also, this clay allows pieces of the pot to absorb any color.
Nevertheless, even after being open, low-fire earthenware clays have a particular tolerance level. So, they might not be able to endure extremely hot temperatures, and repeated exposure to such high temperatures can result in crazing and cracking of the pots made out of this type of clay.
Stoneware clay is another form of clay that can be fired at comparatively higher temperatures, thus making it feasible for being used in pit firing. Also, these clays are considered to be non-porous. So, it may or may not require glazing. However, it is opaquer than porcelain and is often said to be partially vitrified. The three types of stoneware clays mostly used for the pit firing process are as follows:
High-Fire Stoneware Clay: This stoneware clay produces many durable wares through pit firing. It mostly reacts favorably while being exposed to high temperatures.
High-Fire White Stoneware Clay: This type of clay contains a small amount of sand to increase its workability and strength, which makes it slightly open while being pit-fired.
Midrange Stoneware Clay: This clay is similar to white stoneware clay in terms of workability and performance, thereby minimizing firing costs. The low-firing temperature and oxidation atmosphere make it tolerant to thermal shock. Thus, it can act as a suitable clay for being used in pit firing.
3. Raku Clay:
Raku clay contains a lesser amount of grog; hence, it is more suitable for being hand-shaped than getting thrown on the wheel. This clay is quite porous, which makes it good enough for low firing temperatures. The favorable characteristics of this clay make it one of the best elements to be used in pit firing.
Also, if gone through history, the fired raku pieces were initially removed glowing hot from the heated kiln and were allowed to cool down in the open air. However, in present times, the wares are now fired at high temperatures. After the pieces get removed from the pit-fired set-up, they get immediately placed in an open-air container with combustible material inside. However, pots might be returned to the kiln to get re-oxidized, if appropriate firing results are not obtained.
Similar to earthenware, raku clay also has some heat tolerance level, and that is why excessive firing can weaken the entire structural integrity of the pot. Clay potteries exposed to thermal shock innumerable times can even break apart in the kiln, causing massive accidents. Also, since temperature changes are quite rapid during the firing process, make sure that the clay bodies for raku ware are capable enough to bear the significant thermal shock. A high percentage of grog, kyanite, or quartz can be incorporated into raku clay to reduce thermal expansion and add more strength to it.
Porcelain clay is renowned for having some degree of transparency. They are considered to be vitreous and pure which provides them with good quality and durable inherent glaze. The properties linked with porcelain clay include a considerable amount of toughness, hardness, strength, resonance, transparency, low elasticity & permeability, and high resistance to thermal shock & chemical attack. Additionally, it is its resistivity that makes porcelain clay porous and hence, suitable for pit firing.
Porcelain is divided into three categories: bone china, hard paste, and soft paste. Soft-paste porcelain is comparatively weaker than the original hard-paste variant. Thus, it doesn’t require very high firing temperatures to open up the pores and get pit-fired.
Now, we have given a detailed analysis of the pit firing characteristics of the aforementioned clays. Let’s have a look.
|Low-fire earthenware clay
|Reddish or white
|High-fire stoneware clay
|Light grey, brown, and medium grey
|1,100o C-1,300o C
|High-fire white stoneware clay
|1,100o C-1,300o C
|Midrange stoneware clay
|Light grey, brown, and medium grey
|1,160o C-1,225o C
|900o C-1,000o C
|1,200o C-1,400o C
So, if you are thinking of giving pit firing a try, then you can use any low-fire pottery clay, like Bastex 5 lbs Low Fire Pottery Clay; this clay is ideal for beginners as well as professionals.
Hence, these are some of the best clays that you can use for pit firing. While almost any type of clay can be pit-fired, one must practice it with proper care. Thus, it is always better to use relatively opened-up clay having some amount of grog in it. Also, it is necessary to keep the cone size in mind before buying the clay. We hope this content will be beneficial for you.