clay and oil on fire - porosity

[GlisGlis]

my second try with self refined clay.
I had some left from first try and wanted to make an oil lamp vaguely based on the Lucerne (etrurian or roman)
I did not use a mould and the shape is very freely influenced tough.
tha lamp survived firing in an old stove (no idea of the temperature reached)and has been lit and  burning for 2 hours (cotton wick and used frying oil)
the problem is the object is pretty porous. once left a night long full of oil the oil soaked the clay making a thin patina on the outside. It is still usable but is not nice to see or handle
Is that caused by the crudely refined clay or by the low cooking temperatures? or both?
Self made clay is indeed a great toy. Now thinking to make an ocarina.

[nclonghunter]

Interesting and cool idea....I can not answer the firing question but if it "rang" when tapped after firing then it was well fired. In pots that are going to be cooked in or that hold water the inside is burnished with a smooth stone or polished antler. This aligns the clay and compacts the molecules making it way less porous. May be hard to do with a oil lamp and the small openings. My next thought would be to heat the clay and pour in a beeswax to coat and line the inside. If it only burns at the spout then the remaining insides portion should stay cool and the wax coating intact thus preventing it from leaching through the lamp. If you try it, let us know....

[Pat B]

I'm not sure but I'd think you'd need a glaze to make the clay nonporous. You could seal it with pitch or beeswax to help keep the oil from sinking in.

[DC]

Low fired clay(terracotta) is porous. I'm not sure if different clays have different porosities. High fired are(ironware) are basically glass and are watertight. I think that some clay will not take high fire but someone else will know that. For example, the clays around here will make good terracotta but if you try to high fire them they just melt or burn kinda. 

[Zuma]

One way you can test your future lamp would
 be to fill it with water and see if it bleeds through.
It will dry and then you could seal it like others have said.
Native Americans used the porosity to cause evaporation
that cooled the water in the pot. The higher the firing temp
the tighter the material becomes. (shrinkage??)
Zuma

[GlisGlis]

thankyou all for your replys
@nclonghunter yes after firing it sounds good. Did not know the burnishing technique and i'll apply on pot with enoug big entrances.

@pat  are there glazes usable on diy fires or ovens? kinda primitive? or i need to use a high heat 'modern' oven?

@DC thanks in fact I should refere to it as terracotta

@yes Zuma thankyou for the reference. I also have an idea to reproduce a water container made from ages in Sicily (Italy) named Bummolo that uses the same principle. let evaporate some water to lower the temperature

I may try the wax proofing. Do you know of any interaction oil / wax (like oil dissolving wax or making poisonous gas?  )

[Pat B]

I think glazes are high fire.

[mullet]

I think if you apply wax you will just end up burning it as a fuel also.

[Zuma]

I did one of my pit reduction fired pots
with an external burnish. It has no liquid
transfer from the inside.
You might give that a try.
Now I am not sure how much the reduction
plays a part though.
Zuma

[GlisGlis]

I did one of my pit reduction fired pots
with an external burnish.

yes I see.It could work to burnish only the outside
how exactly is achieved pit reduction fire? sand covered pots and fire on top?

[Zuma]

yes I see.It could work to burnish only the outside
how exactly is achieved pit reduction fire? sand covered pots and fire on top?

Glis
I dug a hole in the ground. I put a thick bed of grass in the bottom.
I filled the pots with straw and horse poop. (pine bark etc. will work as well)
I then filled the pit with dry grass and poop.
I then covered the pit with hardwood logs. I covered them with small
limbs and brush to start the fire. When the logs caught fire I then covered
the pit with a sheet of metal roofing and let the fire burn out over night.
The reduction occurs because of the lack of oxygen during the burn.
I have also used my small kiln. I put the small pots and beads in a covered
cast iron kettle with horse poop/grass inside and then fired them.
I am sure the covered kettle method would work very well in a small pit too.
This would work well with small lamps like yours. I would do it with plenty of ventelation because of the burning organics.
Now these two methods result in very black pottery.
Look into the pit fire thread on page two. "I built some stuff to fire."
Zuma

[GlisGlis]

thankyou Zuma
wondering how my father could react if he finds me filling is antique stove with horse poop 

[Zuma]

Glis don't fill the stove
Look, just dig a small hole some where.
Get a steel container or cast iron that you
can close for the most part. (small vent hole).
Fill the hole and container with organics that are dry.
Of course your lamp/lamps are in the container.
Cover the hole with some substantial wood and on
top with some limbs and brush for kindling. light er up.
You can make char cloth the same way.
You could stuff the container with cotton cloth instead of
poop,etc. Two birds one  shot (container)
Zuma

[GlisGlis]

thankyou Zuma
I think i got it.
poop in the stove was just a joke
.... or not