The raw clay wouldn't be vitrified, so it would be porous and would seep or leak. A glaze is a glass, more or less, and to get the silica to melt you need a flux. There are different fluxes for different temperature firings, but suddenly things are getting a little more precise. Without substantially levelling up the kiln tech and design you'll be at best low-fired and probably using lead as a flux. E.g. terracotta and earthernware.
Early peoples would have used wax or fats to seal pots like these, to make them functional. People do that with modern pit-fired pots too, or use other sealants.
I designed and built my own high-fire kiln, but it uses industrially made light-weight insulating and refractory brick, and gas burners, and I use Orton cones to know when I've hit the right amount of heat-work, and a pyrometer to take temperature readings. But some brave souls make their own bricks, and look at the colour inside the kiln to know when they're at temperature.
Early peoples would have used wax or fats to seal pots like these, to make them functional. People do that with modern pit-fired pots too, or use other sealants.
I designed and built my own high-fire kiln, but it uses industrially made light-weight insulating and refractory brick, and gas burners, and I use Orton cones to know when I've hit the right amount of heat-work, and a pyrometer to take temperature readings. But some brave souls make their own bricks, and look at the colour inside the kiln to know when they're at temperature.