# Air bubbles when casting boiled sugar



## westrock (Dec 5, 2012)

I'm casting boiled sugar at the hard crack stage into silicone open faced rubber molds. I am experiencing bubbles in the castings. The bubbles are small and seem to form right on the surface of the mold. The top of the castings are crystal clear with no bubbles. I am boiling the sugar in a copper pan; at 300 degrees F the pan is put in cold water to stop cooking and then placed in a 250 degree oven for 10 minutes to help dissipate the bubbles. I'm not sure if the bubbles are trapped when the sugar syrup is poured into the molds or if the air bubbles are within the syrup and perhaps the syrup should be put under vacuum to draw the air out.





  








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westrock


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Dec 5, 2012


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## westrock (Dec 5, 2012)

I just read this statement on Kerekes Bakery supply website:

"To create glasslike, bubble-free clarity in your sugar/isomalt, pour against a thick vinyl surface. Unlike many silicone surfaces, vinyl will not cause sugar to bubble."

So now I'm thinking that it's possible that the type of silicone rubber has a bearing on the bubble formation in the molds. However, I have no idea what physics or chemistry has a bearing on the formation of bubbles in boiled sugar which is poured into silicone molds.


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## michaelga (Jan 30, 2012)

If I recall correctly  

Silicone molds are 'micro-porous' thus there is a bit of air trapped in the materials nooks-n-crannies (this is what helps make them non-stick).  

When the hot sugar hits the mold the air is trapped between the cooling sugar and the mold wall.   As the air warms up it expands and creates the bubbles.

I believe that there are specific silicone molds that are smoothed out and appear to be very slick and shiny, not dull or matte, they should help.


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## chefedb (Apr 3, 2010)

It could also be in the way you are pourinig. To far away from the mold. Other thing that may help try putting the mo;d on a sheet pan or 1/2 pan and tap the pan.


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## westrock (Dec 5, 2012)

> Silicone molds are 'micro-porous'


That makes sense; thank-you for that. I noticed that the sugar has very few bubbles when poured onto a Silpat mat. A Silpat mat has a very smooth glossy silicone surface texture with probably a minimum of porosity. Next step would be to submit the liquid silicone mold material to a vacuum to eliminate air before pouring over the subject, and then cure the silicone in a pressure chamber to squeeze any air bubbles down, so no porosity forms.


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## michaelga (Jan 30, 2012)

you're making the molds?

do tell...


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## westrock (Dec 5, 2012)

MichaelGA said:


> you're making the molds?
> 
> do tell...


I'm experimenting with boiled sugar in silicone molds to understand if it's possible to produce a quality product. I've made silicone molds for years but have limited experience with boiled sugar.

As an aside......I tried heating up the molds but that made the bubble problem 10 times worse. I tried oiling the molds and that was an improvement. I have also noticed that toffee casts quite well in silicone; that may have something to do with the butter acting as a lubricant, or changing the surface qualities of the mold in some way.


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## michaelga (Jan 30, 2012)

Cool stuff I never would have thought molds could be made at home... never really looked though.

Let us know what you figure out, interesting stuff.


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## minas6907 (Aug 14, 2012)

Hey westrock, where are you getting your silicone? I found a few places online, and was just looking around at prices. I'm glad I saw this thread though, I definitly never though of having air bubbles in pieces cast in silicone, I've cast plenty of times using metal bars, but never silicone, I didnt think excessive air bubbles would form. I wanted to make a silicone mold for an event next year, just for some letters cast in sugar, but I now I'm almost thinking of just using a sheet of neoprene. Would still like to know your source for silicone, if you got a good one.


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## westrock (Dec 5, 2012)

Minas6907 said:


> Hey westrock, where are you getting your silicone?


Minas,

There are lots of sites online for Silicone, one of the largest is Smooth-on. Prices vary so it pays to shop around and keep an eye out for "food grade" silicone. The best silicone that I've found is made by General Electric; the 664 series silicone which is food grade. Smooth-on and some of the other brands tend to deteriorate quickly if you don't use them right away. As far as casting sugar it seems that the most important factors revolve around how the silicone is prepared for mold making. If the silicone is de-aired using a vacuum chamber before pouring and then left to set in a pressure chamber you get better results. Both of these processes help to eliminate micro porosity within the Silicone. I never considered porosity and air pockets until MichaelGA pointed it out. Commercially made silicone molds are probably done using vacuum and pressure but it's hard to do if you don't have the equipment.


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