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Slumping Bottles

Slumping bottles in a kiln is not difficult.  It does, however, require a knowledge of basic kiln-forming principles and an understanding of both thermal shock, annealing, and devitrification.  If you are not familiar with these terms, then spending a few moments with the Warm Glass tutorial will help familiarize you with the process of heating glass in a kiln.  Click here to go to the tutorial.
 
The basic process for slumping a bottle in a kiln is as follows:

1.  Clean the bottle thoroughly and allow it to dry.  Some people also apply a devit spray to the bottle to help prevent devitrification.  This is recommended for most blue and amber bottles and is also helpful for other colors as well.

2.  Cover your kiln shelf with either fiber paper or kiln wash. 

3.  Lay the bottle in the kiln on it side.  It's not essential, but some people place a piece of wire in the neck to form a wire loop which can be used to hang the bottle after slumping. (20 gauge twisted copper works well.)

4.   Fire the kiln to 1100F and soak for 10 minutes.  The rate of temperature increase should be from around 500 degrees per hour.  Some people fire as fast as 800 degrees per hour, but be aware that the faster you fire the more likely the bottle is to crack from thermal shock.  The purpose of the 10 minute soak is to allow the temperature of the glass to equalize and to all reach 1100F.

5.  Fire the kiln at 250 degrees per hour to 1300F, then fire as fast as your kiln will go to 1475F.  Hold the temperature constant at 1475F until the bottle has slumped to the degree you want.  Usually this takes around 10 minutes.

6.  Cool the kiln to 1100F as fast as possible.  You may need to flash vent the kiln to speed cooling and to help prevent devitrification, but some people slump bottles with flash venting.

7.  Anneal.  Some people accomplish this by simply letting the closed kiln cool naturally.  This approach will work if your kiln cools slowly enough through the annealing range (for bottles, roughly 1050F to 850F).  You would want to cool at a rate of 150 degrees per hour or slower.  If your kiln has a controller, a proper anneal soak is highly recommended:  soak at 1030F for 20-30 min then 100 dph to around 850. 

8.  Cool to room temperature.  Most kilns will cool at a slow enough rate to avoid thermal shocking the bottles by cooling too fast.  

 

This suggested firing schedule is adapted from the one used by Judy Schnabel.  It should be modified for you and your kiln.  Some possible modifications are:

1.  Slump at a lower temperature (or soak for less time) if the bottles are slumped flatter than you like.  Increase the soak time to flatten the bottle further.

2.  Slow down the heating or cooling rate if the bottles crack while in the kiln.  If they crack soon after emerging, then the problem is most like improper annealing.

3.  If kiln wash sticks to your bottle, then try slumping to a lower temperature and soaking longer.  In general, the lower the temperature the less likely you are to have problems.

4.  Larger and thicker than normal bottles will require slower firing rates and longer annealing soaks.  

Finally, a couple of tips on removing labels prior to firing the bottle:

1.  Try heating with a hair dryer or steamer.  Most labels are affixed with hot glue, so reheating the glue usually allows the label to slide off.  

2.  Mineral spirits can be used to remove any residual glue from the glass.

3.  If you don't want to save the label, just soak the bottle in warm water, then remove.

There is more information on the topic of slumping bottles in the Warm Glass archives, which may be searched at: http://www.warmglass.com/cgi-bin/wgarchive.pl?search

Enjoy.