[Cob] a cob bathtub

[Amanda Peck]

I don't know.

The fact that your grandfather boiled it to put in oil paint is a good sign. 
  Not so good a sign if he only used it on the roof.  How about furniture, 
or was it too slow drying?

It would be good to know a substitute for the more available in our local 
hardware store versions--linseed or Danish (tung mixture) oil.

Three kind of bad things on boiled linseed oil.

One is that if you're starting with the raw linseed oil it's a bit tricky to 
heat to the right temperature--nowhere near boiling, if I remember the 
instructions.  My mother (the art student) told me that if you got it too 
hot it exploded, and she never wanted to hear about me trying it.  Everybody 
else seems to say, "You just have to be careful." I consider those pretty 
bad words for experimenters and beginners.

Do those warnings go with sunflower oil as well?

Second, boiled linseed oil has a really bad reputation for spontaneous 
combustion.  Be very careful with any rags, oily t-shirts and the like.  
I've heard one story of one house project that burned down twice because of 
spontaneous combustion.  Not sure if that one is completely true, but I'm 
pretty sure about another house that went up in flames once.  This one, with 
details published widely, was from a t-shirt left on the freshily oiled 
deck.

Third, and why my local furniture people have given up on it (in favor of 
the even more expensive--imported--Tung Oil) is that sometimes it is slow 
drying.

Sticky furniture and floors are bad enough. Sticky bath-tub, no thank you!  
Not sure I'm one of the proponents of cob bathtubs anyway.

Here a gallon of boiled linseed oil, industrial grade, maybe with driers 
added, is almost certainly cheaper than a gallon of--food grade--sunflower 
oil.  I don't think I've seen industrial grade here, although I'm sure it's 
available, probably in 55 gallon drums (close to 250 liters).

So the best I can say is "try it" on a small sample and tell us.  (A 
square--20-30 mm deep, maybe a quarter or a third of a meter on each side)

Somebody knows this!  Tell us!  How do we add oil to a floor, plaster--or 
bathtub--mix?  Add it to dry sand, mix well, and go from there?  How much do 
we add? something on the order of a cup--around .25 liter--to a 5-gallon 
(around a hundred liters) pail of sand? I've never tried it, and don't have 
a book handy--it might be quite a bit less than that, more like a cup to 
three buckets of sand, and one clay.

Using it on top is easier, and you probably want to do it this way even if 
you did use oil in the mix.  Just wipe it on.  Mixed with mineral spirits if 
you have it.  Let dry, repeat--probably three coats in all.  Then leave out 
in the rain for a couple of months!
..................
Valdimir wrote:

does somebody have experience with using boiled sunflower oil
for this? I live in Ukraine, and lineseed oil is rare in our
country now (while sunflower oil is the most common). My grandfather
boiled sunflower oil to dilute oil paint for the roof. May it be good
for the floor?