Potato in Glass

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potato in glass

Dipping a potato in a hot glass furnace may seem like an odd thing to do. This is actually an old glass factory trick. This little known trick helps with melting clean, bubble-free glass… And there is some actual science to back this up.

 

Melting a pot of glass

We fill the glass furnace with the raw materials for glass, known as ‘Batch’. Here is a link for more information and a video about what is in our glass.

The cycle for melting glass is known as charging… And putting batch into the furnace is known as – “throwing in a charge”.

It generally takes all day to fill the furnace, throwing in more batch as the previous charge begins to melt.

We melt the glass at a very hot temperature of 24ooº degrees.

The raw materials begin to melt together. Small gassy bubbles, known as ‘seeds’, form in the glass during the melting cycle.

These tiny bubbles or ‘seeds’ will rise to the top of the glass, dissipating away. Getting the seeds to rise to the surface is helped along when the furnace temperature is lowered to 2000º degrees. This process is known as the ‘squeeze’.

However, sometimes these seedy bubbles can’t make it to the surface before the glass reaches the lower glass blowing temperature. The glass is no longer viscous enough and the bubbles become trapped in the glass.

Potato in glass

Seedy bubbles is one of the reasons dipping a potato into the hot glass during the melting cycle is helpful.

The potato has a lot of moisture. This moisture very quickly turns into steam. This volatile burst of steam helps mix the molten glass components.

The steam also creates very large bubbles. Large bubbles have a much easier time reaching the surface during the ‘squeeze’. As these larger bubbles rise, they incorporate the tiny seed bubbles.

This gives us a better chance for clean bubble-free glass to make our glass sculptures.

I have heard of people using other things, such as apples, for this process… But a potato in glass is my choice of fruit or vegetable.

By Bernard Katz

 

10 Comments

  1. Fascinating information! Who knew? Also, wonderfully produced video. Your family’s creativity knows no bounds!

  2. That was fascinating. Thank you for the information

  3. i imagine the density of the potato helps make it the vegetation of choice. it was a little hard to tell, but it seemed like the potato was still in tact once you pulled it from the furnace, ever lost one while taking a dip?

    • one more thing! at what point in the charging cycle do you prefere to “do the potato” ?

      • That is a very good question with some debate among glass blowers. After the last charge, the furnace is set to run at high fire temp for about 5 hours… After which, the furnace temp will ramp down to our ‘working’ temp. I personally prefer to potato the furnace after an hour or two into the high fire. Basically, when the glass is at its most viscous.

    • Hi, you are right about the density. I have heard of people also using things like apples. Because the potato is dense and sturdy, it stays intact pretty well. I did have a fellow glass blowing friend tell me about how he had a potato that got loose… He said it was a struggle getting it out because it was dancing around on surface of the glass like crazy. :)

  4. Thanks for the great info! By any chance would you know if this technique could be helpful for fighting cords in a cullet melt?

    • Generally speaking, yes. Cords are the other main reason I use a potato. I am using mostly batch, and a thin layer of cords can form between each melted charge as the furnace is being filled. The potato helps break up those “charge” layers… However, because cords form when the fluxes in the glass begin to burn out, emptying the furnace completely between charges is almost essential. Otherwise, old cordy glass will just get mixed around with the potato. Also, if your cullet has been melted over and over without the introduction of new material, your just charging cordy cullet.

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