## 4 Discussion
Increasing vitrification and reducing porosity can create impermeable vessels for the storage and transport of liquids [74]. In contrast, greater porosity can be useful for lower-fired cooking vessels that often contain a high percentage of polycrystalline components. The crystals can expand on the cooking fire without exerting pressure that might crack the vessel because the many pores absorb the strain [1]. Micro-CT imaging is a good approach for building more data regarding the relationship between total volume porosity and firing temperature for various ceramic recipes and functions. The percentage of the total volume porosity that is accessible to the surface is one indicator of the use properties of a ceramic. For example, pores accessible to the surface promote evaporation that can help keep the contents of water storage vessels cool [1]. If movement of liquids to the surface is undesirable for certain uses (such as serving vessels such as teacups, or for vessels intended for the transport or long-term storage of liquids) potters can control raw materials, fabrication methods, and firing temperatures to control the ceramic body as well as add slips or glazes to one or both surfaces. While some isolated interior pores form due to the burn-out of rounded natural carbonate inclusions in clays or of additives such as seeds, additional closed interior pores can develop during firing as vitrification of the ceramic matrix proceeds through a combination of controlled use of raw materials, fabrication methods, and firing temperatures to close off previously connected pores that may have once reached the surface [1; 74]. Most methods for analyzing porosity in ceramics only quantify connected pores, but isolated ones can also impact ceramic properties important to use functions [39; 75; 76; 77; 78; 79; 80], so an advantage of micro-CT and 3D image analysis is that pores not connected to other pores or to the ceramic surface are analyzed [38; 51; 81]. Table 2 and Figure 6 show a clear relationship between firing temperature and the amount of porosity that reaches a ceramic surface versus pores confined to the interior. At 900 \({}^{\circ}\)C, the ratio of surface/interior pores is 20.3; ceramics fired between 1000-1150 \({}^{\circ}\)C show ratios ranging from 8.3 down to 1.1; and above 1150 \({}^{\circ}\)C, the ratios are all below 1 (0.9-0.3). Ceramic scientists have noted that a material with many small, closed pores will conduct heat more slowly, while a material with large, connected pores open to the surface  
will increase thermal conductivity because hot gases can pass through the ceramic body [1]. A higher percentage of connected and surface-accessible pores would be advantageous for low-fired cooking wares, to allow heat to permeate the ceramic body evenly and completely. On the other hand, small, unconnected, interior pores are better for a teacup, to keep the tea hot but not allow the heat to pass through the ceramic wall and burn the hand of the drinker. This would be another variable useful to pursue in further research, including selecting for comparison ceramics of various known functions as well as firing temperatures. Archaeological scientists have for many years recognized that changes in average pore size, shape, and homogeneity can provide evidence to infer original firing temperature [1; 7]. However, as with total volume porosity, the situation is complicated because pore size, shape, and distribution are also strongly affected by the choice of raw materials and fabrication methods. For example, low-fired cooking wares often included organic additives that char and partially or wholly burn out during firing, leaving large pores of quite varying size and shape (see Figure 1, left); little kneading and wedging of the clay can create an inhomogeneous matrix (Figure 18, left); and fabrication methods that involve the application of pressure can create long, parallel pores.