While working on the Ur Digitization Project and the condition assessment of the ceramic vessels from Ur, I often find myself thinking about how they were made. Once in a while I notice features that help illuminate that question. My favorite example of this is 31-16-160, which is described in our database as, “pottery, the largest jar, intact.” Part of the reason I like this object is because I love this description and the grand claim of being the largest jar. It may not in actuality be the largest jar in the whole Museum (I am sure it is not), but it is the largest intact jar from Ur (at least in our collection).
To get back on point, in addition to this object having a great description it has some interesting features that provide clues about its manufacturing, specifically how it was fired. If you look at the shoulder and rim, you can see that they are distorted. The color in this area is slightly greenish and the texture of the ceramic is also different from the rest of the vessel. This is evidence that this object was unevenly fired so that the top of the vessel was exposed to a different maximum temperature and was over fired.
There is even a part of a ceramic vessel that was next to it during firing that has fused to the shoulder of the jar.
After this object was fashioned from clay, it would have been dried and then fired. Not to go into too much detail but there are many factors including the type of clay, and temper which affect the ideal temperature for firing. In all cases there is a point where the clay grains begin to fuse together and become more like a glass. It is at and above these temperatures where this type of distortion and color change occur.
Throughout history many different firing methods have been used. Two of the oldest technologies are open fires, which are like giant bonfires with pots in them, and pit firings which are very similar but where the potter first created a pit in the ground and often also a low wall around the pit most likely out of earthen or mud bricks. In both methods a bedding layer of fuel is put down first. The pots are then placed on the bedding layer and more fuel is packed in-between and often over the top of the vessels as well. There are many different types of fuels that can and have been used around the world such as dung, bark, and dried plant materials. Combinations of fuel are also often used. The packing fuel can be the same as what was used in the bedding layer or be something different. The use of these methods likely continued at Ur as at other sites in Mesopotamia alongside the use of kilns.
This jar, however was likely kiln fired and specifically in an updraft kiln. A group of kilns was in fact found at Ur in Pit F. The field photo below shows one of these kilns which during excavation was found to still contain pottery from its last firing.
There have been many different designs for updraft kilns discovered at Mesopotamian sites. Often there are two chambers, one above the other. The upper chamber holds the ceramics and the lower chamber, or firebox, is where the fuel is placed. The top of this type of kiln has some type of ventilation or opening at the top which draws the heat upwards through the kiln. This is why these are called updraft kilns (see image below for an example from Habuba Kabira, Syria). Updraft kilns can reach higher temperatures during firing than pit firing or bonfires. However, the kiln temperatures can be very uneven with the highest temperatures reached at the bottom of the kiln close to the fire box. Furthermore, uneven kiln loading of the pottery can create heat pockets. As a result over firing of ceramics is common in updraft kilns. This jar was likely near the firebox or a heat pocket.
If you look at the bottom of this jar from Ur, you can see circular impressions. These are likely from the rims of the ceramics around the jar during firing.
Below is a possible reconstruction how this jar and others like it may have been arranged in the kiln during firing assuming it was fired with vessels of a similar form.