The question was whether
mineral oil or Sculpey diluent is best to use with TLS to thin it a bit. I did a test - as scientific as I could do it with my lab conditions. I needed to get equal amounts of TLS and then add equal drops of mineral oil and Sculpey diluent to each batch of TLS. I used the film canister tops as
I knew I could put them in the oven. (The tiny lip melted on both lids.)
Also I was checking to see if either made more of a smell than the other, or
if either was stronger.
I made three separate batches. First I filled the
containers with TLS, and added three drops of mineral oil to one, and three
drops of Sculpey diluent to the the other. I used toothpicks and stirred
thoroughly. I baked the first two in the same oven for the same amount of
time. Imagine my surprise when I looked and saw the TLS with Sculpey
diluent was much more translucent than the TLS with mineral oil. In fact,
the TLS with mineral oil was almost solid white. Here is a picture before
removing from the lid.

The TLS with mineral oil is on the left and the TLS with Sculpey diluent is on the right.
I then removed them from the lids and laid them on a light table. The TLS
with Sculpey diluent allows much more light to shine through and almost no
light comes through the TLS with mineral oil. I could not get a good picture
of this.
I then baked another TLS with the same amount of Sculpey diluent for 20
minutes. Did the same for TLS with mineral oil to see if the smell is
greater, and to see if I could tell, when slightly raising the temps, at what
point the TLS gives off the stronger fumes. I could not tell any
significant difference in the smell, although I think that I saw a little
smoke come off of the TLS with Sculpey diluent right before the time was up.
When I took them from the oven and smelled them, the TLS with Sculpey
diluent had a slightly stronger smell. (I'm not the nose in this
house - but hubby confirmed this for me.)
Then I baked two more and pushed the temps up to 325 degrees for five
minutes. Amazingly, the TLS with mineral oil turned more translucent. Still
not as translucent as the TLS with Sculpey diluent, but it helped.
Last test - in all experiments, the TLS with mineral oil had more crackle
effect in it. Now for the strength. The TLS with Sculpey diluent is more
rubbery. The TLS with mineral oil was harder to bend, and developed a crack
when bending exactly (as exact as we can be in this lab) the same bend as
the TLS with Sculpey diluent. The TLS with Sculpey diluent just remained
rubbery with no signs of cracking. This is the one where I pushed the
temps up to 315 degrees and the TLS with mineral oil turned more
translucent. In this picture below you can see where the crack is in the one on the left. It appears that it will easily break in half.

So much for this! My conclusion - the Sculpey diluent in TLS makes the TLS
more rubbery, and it is clearer, partly because of less crackling than the
TLS with mineral oil. The TLS with mineral oil can take higher temps than
TLS with diluent, and may even need higher temps to become more translucent.
TLS with mineral oil is harder, but not quite as flexible as TLS with
Sculpey diluent.
Let me stress that this is not under extreme controlled conditions so your
experience may vary - but for myself, I think I saw enough evidence that the
conclusion above is a valid one.
I looked at all my samples
again this morning, and all of the TLS with mineral oil is more translucent on the bottom - I suspect this is where the mineral oil settled down while baking. The top is whiter. In the photo below on the left, you can see the bottom and it appears very translucent, but when I turn it over, the photo below on the right, the top is more opaque (this is the same piece which is shown above with the crack in it).


That's it. Hope someone can use this info.
Added Tip:
"The consistancy was one of the conditions that I meant to mention about TLS. I spent over thirty minutes remixing my TLS before this test. I had one can that was so thick in the bottom that I broke three heavy duty tongue depressors trying to remix thoroughly. I ended up using a new container and even it was very stiff one inch from the bottom. So this is one of the conditions that could affect the outcome of the test that I did. I believe that it is diluent of some sort that is separating out of the TLS. My white Liquid Sculpey - not TLS - doesn't appear separated, and it is OLD, probably three years old at least. It is very thick and the same consistency from the top of jar to the bottom."
Safety Tip:
"I would always be careful when using Sculpey diluent. It is somewhat of an
irritant. If I accidently wipe my face with my hands after having used a
few drops in the clay, I have to immediately wash my face carefully. And
don't ever get it in your eyes as it will burn."