...addendum to Conditioning page...

TESTS of STRENGTH and FLEXIBILITY after baking
(with various amounts of conditioning, and different brands, etc)


latest breaking news?! – Feb 6, 2000 (from Sue Heaser):

The softer clays such as Sculpey and Premo do *not* need "conditioning" (for strength alone). This is from the manufacturers' tests and my own tests for my books.
... These conditioned clays show no differences in strength compared to the same clay used straight from the pack...
....It is an urban legend that you need to condition these particular clays for strength. . . In fact, conditioning any clay can incorporate air bubbles (particularly in translucents) and make rolling smooth sheets harder to do.

BUT, the stiffer clays like FimoClassic (especially any older Fimo), and to a lesser degree FimoSoft, need to be worked just until smooth and pliable or you will not get the full benefit of what you can do with clay.

..............FAX from Henry Schnorrer, the chemist at Eberhard Faber....................
Dear Sue,
Thank you for your fax from February 8, 2000. The recommendation in your book (that kneading is only necessary to soften FIMO), is absolutely correct.
FIMO will not be stronger by a long period of kneading.
If you have any further question, don't hesitate to contact me. Best regards, Henry Schnorrer

also

However, any clays which need an exact consistency of color (expanses of solid color which will appear on a broad surface area, for example) may need to be mixed just until there are no detectable lighter or darker patches within the color.

I brought this subject up with Marie Segal and Howard at the Clay Factory yesterday and what Marie said was this.
"When a package of the clay sits for a while, the chemicals in it tend to drift down to the lowest level, which means that they are not evenly distributed throughout the block of clay the way they were when first manufactured.
What the chemist told us is that for the best possible product, these chemicals should be redistributed throughout the clay.
You may not be able to see or feel the difference, or even see it in a bend and flex test, and it doesn't mean the item will crumble or fall apart ... but there is some overall difference in the finished piece which could mean less stability somewhere down the line." So I guess you all can decide if you want to condition or not. Dotty in CA

There were some real surprises for me (after I conducted my tests) ... I had been convinced that the advocates of lots of conditioning were right and it's hard to give up something that's become almost "dogma" in your mind, you know?
.....But, after seeing the brittleness increase with conditioning, and the clarity of the translucent go right out the window, I'm now sorry that I conditioned that whole brick of (Premo Translucent with Bleach) the other day before I left it to leach. :-( ....I'm hoping that the really thin pressing it will get in the pasta machine will break any air bubbles it got from the conditioning. Elizabeth

other Strength Factors

BAKING TIME
Many clayers feel that baking a piece longer than the minimum recommended time willl make it stronger.
...as long as the temperature in the oven never exceeds the correct temperature, most clays can bake for hours with no problems (this can be esp. hard to do for a long time in a toaster oven though.... using a convection oven makes this the easiest)
....(...however pieces with lots of translucent, or which use the lighter Sculpey colors, may darken more than you like --especially if you're not using an "enclosed" baking method.... see Baking > Enclosed for more info)
...I remember one video I have did a similar test..but using only Premo and varying the baking times . . .I baked one at 30 min, one at 1 hr, and one at 2 hrs.. The one baked for 1 and 2 hrs were MUCH more flexible (and therefore stronger) than the one baked at 30 min. Jan

BAKING TEMPERATURE
In general, baking clay at a higher temperature can make it stronger also (even for 5 min. of the total baking time).
...however, it can be hard to do:
......some clays can't tolerate higher temperatures, except for a short time, and will easily darken (translucents, lighter Sculpeys)
.........(Kato clay can tolerate the highest temperatures for normal baking of all brands so far, and without darkening)
......thin clay, or clay pieces with thinner projecting areas can darken
......pieces sitting directly on metal or ceramic can darken (put a few sheets of paper, batting, etc., between)
......pieces located near oven walls, or in hot spots of an oven
..convection ovens bake a little hotter than the equivalent temperature set on a non-convection oven, which can result in stronger clay ... the air ciruclating inside keeps the temperature much more consistent than a regular oven would.... conversely, it's hardest to do a higher baking temp. in a toaster oven because it's small (and has no air circulation)
FULLY CURING
When Polyzine tested the clays they baked them all at 260 degrees and reported that Premo was the weakest of them all (!?). . . .(Dec 2000)
....Well, the reason for that is that the Premo brand MUST be baked at it's recommended temperature of 275 degrees in order to be fully cured and strong. Since that particular clay was not completely cured, it tested as weaker. ....When baked correctly it is one of the strongest and most flexible of the clays. Dotty
(other clay brands have diff. recommended temperatures)

ICE WATER PLUNGING
Although ice water dunking of clay still hot from the oven is primarily used with translucent clays to increase their clarity, some clayers feel that it also increases the strength of baked clays.
...A friend of mine said the it's a scientific fact that the clay will be stronger if it's quenched (in cold water). .....I'll try and get her to do a little write up for us when she gets back from vacation. She has been doing this method for about 5 or so years now and is convinced of this. Geo (Lynda Struble)
...the ice water dunk took only about sixty seconds or so. . . . Dotty
(see more on this technique in Translucents > "Clearest Results > Quenching)

TESTS

(NOTE... KatoPolyclay had not come out when the following tests were performed)
.....Kato Polyclay is definately the strong! ...both Judy Belcher's daughter Maria and my son Jeremy did school science projects testing the tensile strength of several different polymer clays, and the results proved that Kato Polyclay was the most durable and resiliant polymer clay. Lisa P.

 

........Sue Heaser's tests of sheets of baked clays of varying brands for her new book: (2001?)......
(from weakest to strongest)

MANX --October 2001
(I was able to conduct a comparison test on five polymer clays: Cernit, Premo, Fimo Soft, Super Sculpey, and Sculpey III. I tested conditioning, a few basic sculpting qualities like adhesion of clay to clay, and durability. You can find my results at: http://www.gigagraphica.com/poly/polyclay.html
(P.S.) I live in Colorado. The altitude is over one mile high, and it's the high plains, so the humidity is virtually non-existant. That does seem like an important thing to add to my test. )

GARIE --Dec. 2001
Du-Kit, Sculpey III, Super Sculpey, Premo, Fimo Soft, Fimo Classic and Cernit. . . . I used a conventional oven with better heat distribution, this is my home oven which we had use it to bake cake. The clay is non-toxic, I had roll all the clays into a diameter of 6mm thick and baked them for not more than 20 minutes at the various temperatures .
http://www.garieinternational.com.sg/clay/clay_test.htm

ZIGGYBETH (Elizabeth) --Feb 00,
. . . Anyway, here are the results of my tests. Again, no real "results," and more
questions than answers. . .
***One thing I learned for sure is that conditioning the translucent clays is adding air pockets, which interferes with translucence, just the way rutilations break up the clarity of quartz crystal. So from now on, I'll be "conditioning" my Art00 and CFC 06 just enough to get it as thin as I want it.

I had five samples: Fimo from 1994 that was crumbled in a food processor with lots of diluent and then left to sit in a zip-lock bag for about two months, Fimo fresh from the factory, FimoSoft fresh from the factory, Premo fresh from the factory, and SculpeyIII that's at least 3 years old.
---For each sample, I made three batches, one batch with no conditioning, one batch with ten trips through the pasta machine and one batch with twenty. From each batch, I made a bandaid size strip on the #1 setting and one on the #4 Setting
---Baked on the same cookie sheet at 275 for 30 minutes.
---Immediately removed sheets of paper that they were baked on to a cold kitchen counter, and bags of frozen vegetables placed on top, to cool them quickly. (I couldn't just dump them in ice water, or I'd have not known which was which... that was written on the baking paper. ;-)
---I attempted to fold each piece in half back and forth at least twelve times, and noted at which fold # the clay cracked more than halfway through.
---I attempted to tear the piece, which I could not do, in most cases, except at the flexed area. If I could tear it any place but a flexed area, I noted it. (My hands ache! *g*)
In these little bandaid type strips, you could see obvious air layers in all of the "conditioned" and "half-conditioned" clay which had not been apparent before baking.

Fimo 1994 (Classic)
Not conditioned:
#1- grainy, chalky, won't fold in half, cracks upon flexing, tears easily
#4 - smoother, flexes, cracks on 4th flex, tears easily

Half-conditioned:
#1 - folds in half, flexes, broke at 12 flexes
#4 - folds in half, flexes, broke at 16 flexes

Fully conditioned:
#1 - folds in half, flexes, broke at 20 flexes
#4 - folds in half, flexes, broke at 12 flexes

Fresh Fimo (Classic)
Not conditioned:
#1 - folds in half, flexible, tears at flex, broke at 7 flexes
#4 - very flexible, folds in half, difficult to tear, broke at 10 flexes

Half-conditioned:
#1 - folds in half, obvious air layers, broke at 12 flexes
#4 - folds in half, air bubbles, broke at 8 flexes

Fully conditioned:
#1 - folds in half, broke at 10 flexes, difficult to tear
#4 - folds in half, broke on 25th flex, could not tear

Fresh Fimo Soft
Not conditioned:
#1 - Broke at 23rd flex, folds in half with difficulty
#4 - Broke on 9th flex, stretches

Half-conditioned:
#1 - Broke on 8th flex, difficult to fold in half
#4 - Broke on 28th flex

Fully conditioned:
#1 - Difficult to fold in half, broke on 14th flex
#4 - Broke on 29th flex

Fresh Premo:
Not conditioned:
#1 - Difficult to fold in half, broke on 36th flex, stretchy
#4 - Broke on 27th flex, stretchy

Half-conditioned:
#1 - Finally began showing some cracking at 38th fold, very difficult to fold in half
#4 - Did not break in 50 flexes, stretchy

Fully conditioned:
#1 - Obvious air pockets, broke at 8 flexes
#4 - Shows cracking at only 2 flexes, broke at 4

Sculpey III (at least 3 yrs. old)
Not conditioned:
#1 - Cannot fold in half, broke at 18th flex
#4 - Broke on 12th flex

Half-conditioned:
#1 - Cannot fold in half, obvious air layers, broke on 6th fold
#4 - Broke on 22nd fold

Fully conditioned:
#1 - Cannot fold in half, broke on 13th flex
#4 - Broke on 10th fold

Whew~! Anyone else? Zig

LATER: Hi, Diane, I wish I had time to be really thorough... I should have tested another batch using the "rolling and twisting," too. Ah, well... my hands ache enough, today, as it is!

As to your results, they were quite eye-opening.
There were some real surprises... I was convinced that the advocates of lots of conditioning were right, and it's hard to give up something that's become almost "dogma" in your mind, you know? ;-) But, after seeing the brittleness increase with conditioning, and the clarity of the translucent go right out the window, I'm now sorry that I conditioned that whole brick of CFC06 the other day before I left it to leach. :-( I'm hoping that the really thin pressing it will get in the pasta machine will break any air bubbles.

Some questions: Did you keep a record of which colors you used for each brand of clay
Old Fimo - terra cotta, Fresh Fimo - golden yellow, Fimo Soft - fluorescent pink, Premo - white SIII - Translucent

Do you think the quick cooling of the clays had any effect?
I'm not sure... this is another area that I should have included, to be really thorough.

(and was there any particular reason you wanted to do that?)
Yes, I've heard that the ice-water bath increases strength in cured clay, and I've just gotten into the habit of cooling things very quickly. I probably should have done another batch and let it cool in the oven, like Irene.

So many factors, not enough time! *g* Interesting, anyway....Elizabeth (Ziggybeth)


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16 Feb From: "scott & irene" Subject: Strength results, sort of
I tested three batches of Premo -- two of fresh clay, a third of clay that is at least a year old.

unconditioned clay:
-- Maggie reported the unconditioned Premo (sheet of #3 thickness) was very brittle.
-- Dotty's unconditioned Premo did not break, but did tear after a few twists and turns. (thickness not stated)
-- My unconditioned *old* Premo snapped easily at both #1 and #3 thicknesses.
-- My sample A of fresh unconditioned Premo snapped at #1; at #3 it flexed several times before snapping.
-- My sample B of fresh unconditioned Premo flexed at both thicknesses many times and did not break.

halfway conditioned clay:
-- Maggie's partially conditioned Premo had some flex before breaking, but she still considered it brittle.
-- My par-conditioned *old* Premo snapped at #1; flexed twice then snapped at #3.
-- My par-conditioned fresh sample A snapped easily at both thicknesses.
-- My sample B snapped easily at #1 but remained very flexible at #3.

completely conditioned clay:
-- Maggie's conditioned sample was very flexible.
-- Dotty's fully conditioned Premo was almost impossible to break or tear.
-- My very conditioned *old* clay snapped easily at #1 and #3.
-- My sample A of fresh, fully conditioned clay flexed only once at #1, then snapped; #3 thickness snapped immediately.
-- My sample B of fresh, fully conditioned clay flexed quite a bit at #1, then developed a fracture line. At #3 thickness it remained extremely flexible.

…Maggie and Dotty had the results I had expected . . .My own results were not strictly in keeping with my hypothesis, though. How can it be, for example, that my unconditioned sample B had more flex at #1 thickness than the same clay partially conditioned (which snapped)?

Other things that probably affect strength:
-- the age of the clay (all my samples of old Premo snapped, even the well-conditioned ones)
-- the cooling -- Maggie plunged hers into cold water to cool quickly; I let mine cool in the oven.
-- the baking time and temperature. Sure, we all said 275 degrees, but I have three thermometers in my oven and no two read exactly the same; I average them out. Time? Maggie did 25 minutes, I did 35 minutes.
-- method of conditioning -- if all clay is run through the pasta machine in the same direction every time, is it stronger or weaker or the same as a clay that is turned 90 degrees with every pass through the machine?
-- and heck, the batch of clay from the factory! Suppose someone puts a leeeetle more plasticizer or pigment or some other ingredient in every other batch on any given day.

Anyway, no answers. In fact, it just raises more questions for me.

Personally, I'm gonna keep on conditioning. I have to, as all the colors I use are custom mixes (except for black and white). Oh well. …Irene in western NC
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from Donna : "Also, thought about this in the shower....size does matter! Compact, small pieces (this is the usual scale of European work) will always be more resistant to breakage, I think. We're trying to work bigger (at least I am) and I think it makes a difference."

You know, when I switched to the convection oven, I felt that my clay came out stronger? And it does depend upon what color clay we're talking about. I finally used up some old Fimo-esque white Premo that was a pain from start to finish. It needed a lot of diluent to condition it and I don't think it was as strong as the other Premo colors I've used. Until recently, I was not a big user of white, so maybe I don't really know what I'm talking about.Bottom line for me, I want fresh clay (I don't mind leaching it if necessary) and I do believe in conditioning it until it "feels" ready. With Premo, that stage is when it's warm, smooth and elastic. Jody

I had a thought while reading the test results. Does anyone think that could play a part? I know it may not seem possible, but in cooking we have to adjust some things because we live at 6000 ft. I wondered if we need to adjust the oven for the clay? Just an idea Jeannetta
Now that IS a thought! Water boils at different temps at different altitudes. We live at almost sea level so that could make a difference from someone who lived at a higher altitude. Hummm. Anyone else have an idea on this? Dotty in CA

From: "Jenny" , Subject: Re: Strength results, sort of
.I did a test!!! But it was on my Aussie brand Modelene....and I almost wasn't going to bother sending the results since you guys don't use it<G>
I just did two tests...one using "barely" conditioned clay and rolled #2 on an Atlas, and one fully conditioned at #2. They were both baked at the recommended temp for 30 mins. Age of the clay was about 8 months I'd say....
Silly me didn't do a totally UNconditioned piece...but I will today..and will let you know. Both pieces seem to me to have the same strength and flexibility. I bent them both until they were touching without either breaking or showing any sign of stress….Jenny in OZ

????I do think we will find we get variations in pieces we bake and a lot is down to chance - position in the oven, length of baking, temperature etc. To do a proper test you would need to bake lots of samples and take the mean. I think the above just shows that it is not the kneading that affects strength, it is the baking and conditions at the time in the oven. Sue
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OTTERFIRE: Last night i tried a more scentific approach (dont critize it doctor) and did 2 batches tested sculpy, premo and fimo....
i conditioned all the clays the same way, cut 3 peices of each one left flat, one i folded like a flower and made a bead out of the other one. so i had nine samples for each test.
---the first was the control...baked regularly and the second was baked the same way and after 15 minutes, i turned the oven up and then once it hit 300 i turned it off. i opened the oven door on both to allow them to cool. here are the results:

sculpy
--there was a slight darkening in color, but not charred.
--the strengh of the sculpy sample was increased slightly..the 300 peice didnt snap like the control, it cracked on the edges first and then snapped.
--i noticed no difference in 2 sculpy beads.

premo
--there was no noticable color change in these samples.
--the strength was increased. i can double the premo 300 sample over and the control snapped. ---there was a whitening on the fold line in the 300 sample but once the stress was removed the white disappeared.
--there was a slighly more *rubbery*feel, but i dont know if that is the proper word.
--i noticed no difference in the beads.

Fimo
here is where i noticed the most difference
--there is no visible change in color
--i can double both samples over but the control started to tear at one edge, the 300 did not. both left some lighter colored stress marks…
--on the fimo samples i noticed a remarkable (mho) difference in feel. the 300 was *smoother* more plastic feeling. When the 2 samples were folded, the control didnt have the sheen that the 300 did, it almost appeared to have a finish on top of the clay...
So there you have it. I am not sure if this would make a huge difference for all projects, but i certianly think from now one a fimo project will get this treatment. otterfire

I will probably make a few of you a bit crazy saying this.....but...since reading all of the posts on conditioning clay and being concerned about some cracks that I have found in my beads... I began conditioning my clay only enough to make it pliable and to be able to 'rope' it. I cut the size I want, roll it in my chosen bead roller and bake it at 260 degrees regardless of whether it is Premo or Fimo (only use these 2 brands right now). In 20 minutes, my bell rings, and I promptly take the cured beads and plunge them in cold, cold water until the beads are cool. Since I have been doing this --for just the last 10 days or so-- I have found NO cracks in any of these beads.
Dianne C.

(275 F) degrees for 25 mins. . . . I bake Sculpey III at about 250 and find that is fine. It does not discolour at that temp - and I have just done all these tests on translucent. . . Translucent tests - #1 and #4 on the Atlas. Premo 5310 (not the bleached - we cannot get that in UK). As always in my tests, I limit variables as much as I can so:
All baked the same - into preheated oven, out when time up (25 minutes).
No quenching, no sanding, no buffing, no nothing. Sue

Hen Scott (British Guild committee member) --test results:
(Variation is high, but the averages produce very similar results to Ziggy's smaller samples. . .also interesting were the findings re how much stronger Premo is than Fimo Soft.)

I had a go at comparing the strength of pc after different amounts of conditioning . To start with I just flattened a block of clay between my hands to get it thin enough to go through the pasta machine, then put it through once at setting no. 4 to make a long thin piece, then I cut off 10 x 1cm wide strips. Then I cut the remaining piece of clay in half and put one half through the pasta machine 10 more times and cut off 10 more strips, then the other half through 20 more times and cut off the last group of 10 strips.

Each strip was marked to show which conditioning group it was in (low medium or high) then they were baked at 130 degrees C (275) for 25 mins - mixed up on the baking sheet so there would be no influence from position in the oven. Once they had cooled down I covered the mark with a sticky label to make the test completely blind, mixed them thoroughly and numbered the labels from 1 - 30.

To test their strength I just bent each strip back and forth until it split across and could be broken, counting the number of times it took to do this. This number was recorded for each strip from 1 - 30. Once they had all been broken I took off the labels and recorded which conditioning group each strip had been in (low, medium or high - ie once, 11 times or 21 times through the pasta machine).

The first couple of times I did it the results were all over the place and far too variable in all the groups to draw any conclusions. This was with Fimo Soft and the clay just fragmented when I put it through the pasta machine, so even after 20 times it wasn't really conditioned but only just holding together.

The next time I did a minimal amount of conditioning by hand before – just so it would hold together when it went through the pasta machine – the whole block had exactly the same amount. The figures were still quite variable but one could see a bit of a pattern - ie less strength after low conditioning, but not much difference between medium and high – though there was a wider spread in the high group and I was surprised to see a couple of very low figures.

I then did it again with Premo and found little difference between the low and medium group (though again quite a wide spread) and surprisingly, slightly less strength in the high group.
The Premo samples were much harder to break and I had to use pliers! My hands are still recovering!

Here are the figures:

FIMO SOFT:
low - 1,5,1,1,5,1,6,2,2,5 (no. of bends it took to break strip)
med - 7,9,11,6,7,5,11,15,10,8
high - 12,7,7,11,5,11,10,17,3,4

averages:
low - 2.9
med - 8.9
high - 8.7

PREMO
low - 35,9,34,45,42,38,30,43,48,48
med - 23,26,51,28,28,39,36,50,44,53
high - 28,25,27,57,32,46,37,41,20,42

averages:
low - 37.2
med - 37.8
high - 35.5

 

Clearly this isn't very scientific, but my tentative conclusion is that strength is very variable until the clay has been conditioned enough to make it workable, but after that point, further conditioning doesn't make it any stronger.
Fimo Soft needs more conditioning to reach this point – hence the lower figures in the low conditioning group.
Fimo Classic would presumably take even longer.

If we wanted to do a proper study I think we would have to find a more scientific way of breaking the strips as one can't be sure one is applying the same amount of force each time.

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