Why are diyas kept in water before pouring oil in them? What makes diyas absorb less oil after they are being soaked in water? My mother had to light up diyas  (made like earthen pot in which we poured oil, dipped cotton and lit it).
So before she poured oil in diyas, she submerged all diyas in water for a long time and after that kept them out. When they dried up she poured oil in them.
I asked why she kept diyas in water. She told me that in this way diyas will not soak more oil.
But how? After drying up by keeping them out of water, why didn't they soak more oil as compared to diyas which weren't kept in water? What change did water make? (Remember we poured oil after we took out diyas from water and made them dry.)
A diya

 A: Clay pottery can absorb water owing to its chemical nature and porosity.
https://en.m.wikipedia.org/wiki/Clay_mineral
In short, the material composing the diyas is hydrophilic.
As for water and oil aren't miscibile, soaking the diyas before use does indeed prevent the oil to permeates the clay, so it does not get greasy.
Note that for the above chemical consideration, we shall not expect that oil permeates the pottery that much. However the above water/oil immiscibility will certainly keep it clear and is the mechanism you are looking for.
Once soaked, the pottery retains water at its interior. So the fact that it looks dry doesn't mean that it does not contain enough water for the next use. So the soaking isn't necessarily done just prior of lighting it.
What seems more important, is that although refractory, clay is more brittle when dry than when it contains water. Furthermore, water has a high heat of evaporation, so it does mitigate heating. Therefore, soaking the diyas prevent cracking by ageing and use.
Note that soaking for the above reasons is a recommended practice for cooking pottery. In addition to the reasons above, on the stove, a soaked pot can gently release the water preventing an overly dry cooking as well as homogeneously distributes the heat, both leading to an evenly cooked food.
A: Oil and water are both fluids.
They don't have fixed shape and take shape of container . Both oil and water will try to occupy space between clay particles . Once we log diyas in water the interparticle spaces are filled with water . Oil and water do not mix into one another. So oil cannot occupy the space that had been occupied by water . Absorption by definition is when substance get soaked in the interparticle spaces .less oil absorption takes place . .
A: Several diyas are lighted by oil in festivals like Diwali in India to create a standing flame of burning vegetable oil.. To make diyas many pieces are clay fired to a bowl shape with groove provision to hold a cotton wick.. Like porous surais ( earthen pots used to cool water in summer) the pores are filled by capillary action with water which evaporate and cool the water held inside the pot.
When soaked in oil (that has a density less than water, is water immiscible ) diyas serve as water bottomed oil containers because water acts as a impermeable membrane/layer as dish inside surface due to higher density than oil, so oil must float. It  is as if you are using a metallic diya, no oil seeps down through.
An added advantage is that it insulates temperature unlike a  metallic diya that gets hotter during oil combustion and is expensive because some 15-20 numbers are purchased during a Diwali season... to be discarded after a single or couple of use.
For such reason even cooking in thicker rice/vegetable/soup in earthen pots is successful.
