BOTTLE SLUMPING TIPS PART 2: FIRING

In our most recent Bottle Slumping Tip - Part 1, we discussed bottle slumping preparation. In this tip we'll cover firing.

Many of the questions listed here come from customers who purchased our Trio glass slumping kiln. Others are related to the bottle molds we sell. Therefore these questions are somewhat specific to the Trio and bottle slumping, but they actually apply well to many glass slumping situations. You can also refer to our first Bottle Slumping Tip, #92.

WHAT'S THE BASIC FIRING SEQUENCE FOR SLUMPING BOTTLES? WHAT'S THE GENERAL STRATEGY? WHAT'S HAPPENING IN THE KILN DURING THESE STEPS?

First, a disclaimer. There are lots of firing sequences out there you can try. There are different strategies, with much more elaborate firing sequences. This approach has worked well for us for the Trio kiln. Hopefully you will find this a good starting point. Remember, the best firing sequence is the one that gives you the results you want, not necessarily the one you saw in a book or your manual, or was given to you by a friend. If you don't like your results, try changing how you fire.

Here is a nice, simple generic firing sequence for slumping bottles. Let's break it down.

STEP 1:

The purpose of Step 1 is to start getting the bottle hot. Nothing really happens to the bottle below 1100 (when you're heating up). The bottle still has its original shape. A 500 deg ramp rate is about as fast as you'll want to go. Slower is fine, and possibly a little safer for your molds. Any faster though, and you risk cracking the molds due to thermal shock. (When you increase the kilns temperature too quickly, different parts of the mold may be at significantly different temperatures, thus the likelihood it will break. )

The hold time is added at the end of the step, after the target temperature is reached. The length of the hold time can be varied. In this case it's fairly short at 10 minutes. The longer you hold, the more time everything in the kiln has to reach the 1100 deg temperature. If you have a large kiln, with multiple layers of bottles, having a longer hold time (maybe 30 min or more) will help get all of the bottles to the same temperature before proceeding to Step 2. This will help get consistent results throughout the kiln.

STEP 2:

Step 2 takes the bottle to a very soft state. It's starting to collapse in the thinner sections of glass (mainly the middle). The bottom and neck are thicker and will be the last areas to slump. To help keep all sections of the bottle at the same temperature, a 250 ramp rate is about as fast as you want to go. The hold time is typically about 20 minutes (to help equalize the temperature and get the middle of the bottle slumping a bit). With a longer hold time, the bottle middle gets even softer and collapses more readily in Step 3. With a shorter hold time, the middle stays a little firmer and tends to give you a "dishier" look in step 3.

STEP 3:

Step 3 is the slumping step. The bottle will collapse. The general strategy for the step is; get up to temperature, get the slump you want, and promptly move to a lower temperature so the bottle stops slumping. You generally want to ramp at about 300 deg/hr from 1300 degrees (fairly quickly). There is a balance here between the target temperature and the hold time to get the amount of slump you want. I have found that 10 min at 1430 is a good start, but my best firing sequences have been modified slightly from that, depending on that I was firing and what look I wanted the bottle to have.

STEP 4:

Step 4 is the annealing step. If you do not do an annealing step, you may have problems with your bottles cracking once they are fully cooled. To anneal, you want to hold the bottle at a temperature between 800 and 1000 deg for about an hour. That allows the stress that has built up in the glass (from it changing shape) to dissipate. When I first started slumping bottles, I did not bother with an annealing step. The bottles always came out fine. However, I anneal now, just to be cautious (I guess). So unless you can't afford the hour, I recommend an annealing step.

I'M FOLLOWING YOUR FIRING SEQUENCE AND GETTING BAD RESULTS. WHAT'S GOING ON?

Potentially a lot of things. Let's discuss two common ones; bottle variations and temperature.

Bottles: Bottles come in all kinds of different shapes, sizes and glass properties. Thicker bottles will take longer to heat and slump. Thinner bottles the opposite. With different shaped bottles, they will slump differently, at different rates. Different glass may have different melting temperatures. You may find you need to sort your bottles and have slightly different firing sequences for different styles of bottles. Remember, bottles are something you're getting for free, so the manufacturers aren't putting a lot of effort into making them perfectly consistent for us bottle slumpers. I suggest keeping a log of your firing, noting the kind of bottle, how you fired it, where it was placed in the kiln, and the results you got. You may see some patterns that will help you get more consistent results.

Temperature: Keep in mind is that just because your temperature readout says 1300 degrees, the entire kiln interior and all its contents may not be at that temperature yet. All you know is that the thermocouple is sensing 1300. Consider a Thanksgiving turkey. Just because the oven is at 375 does not mean the turkey is ready to eat. It takes hours to heat a turkey. The same situation applies to kilns and bottles; It takes time for everything to heat up. Also your kiln may not be able to heat up as fast as the kiln that this firing sequence was developed on. Or you may have loaded your kiln with more bottles, so it will take longer to heat. You may have multiple levels of bottles/shelving and it will take longer for the heat to penetrate into deepest areas. This means you'll need to adjust some parts of the firing sequence to get better results in your kiln. It's rare that a generic firing sequence will get you the perfect results without some modification.

Note: Remember that the "ramp rate" is the maximum rate the controller will allow the kiln to change it's temperature. If you program the controller to heat faster than the kiln can actually heat, the kiln will simply heat slower than the maximum that is allowed. Unless it's going so slowly that it causes an error, the controller will not let you know this is happening. However it's important that you know, because it effects what's happening in the kiln.

For example, you may want a 500 degree ramp rate, but you may only be getting 350. That slower ramp may mean you'll need a shorter hold time at the end of the step. This is because the kiln has already equalized the temperature, because it took longer to heat up. If you suspect this is happening, you can verify by timing your segments. A 500 per hr ramp from room temperature (70 deg) to 1100 should take just slightly over 2 hours. ( (1100 -70)/ 500 = 2.06 hours.) If it takes longer than that (say, 2:15 or more), you'll know you're not really getting a 500 deg ramp rate.

• How can I avoid trapping air bubbles in my slumped bottles?

  It's hard to completely avoid. The general strategy is to get the rest of the bottle to collapse first while keeping the neck of the bottle open longer. Try firing a little slower, including having a longer hold time in Step 2. Keep the neck pointed away from sidewall heating elements, if possible.

• My slump was pretty good, but I got some sharp, jaggy edges. What do I do?

  You get sharp jaggy edges when the glass gets too hot and begins to slide/pull-down towards the lower areas of the mold. Try a lower temperature (maybe 10 degrees) and/or a shorter hold time (maybe 5 min less) on Step 3 (your highest temperature step). If it's not too bad, and you want to salvage the bottle, knock down the sharp edges with a metal file. It just takes a few moments. I find that bottles with permanent decals tend to do this if the decal edge ends up near the crease of the slump. This is true even if the firing was at a proper temperature and hold time. I have not seen any jaggy edges when firing flat to the shelf, only in curved molds.

• Firing sequences that work well in molds don't look as nice when firing flat to a shelf. Any rules of thumb I can follow to adjust my firing sequence?

  Yes. When firing in a mold, the target temperature is generally a little cooler, with a shorter hold time in Step 3, than when firing flat to a shelf. If you don't make that adjustment, you tend to get jaggy edges. If you like a slumpy (flat) look on your shelf-fired bottles, increase the hold time a bit (try 5-10 min). If firing in a mold, and your edges are curling inward, try reducing the hold time by 5 min (in Step 3). If that makes it too un-slumped looking, then try adding some hold time to Step 2.

• I have a tall kiln so I can fire more than a single layer of bottles at one time. What adjustments do I want to make to the generic firing sequence? Any other considerations?

  Generally speaking, you'll want to slow down the firing so the heat has more time to reach all the areas of the kiln. This is particularly true in Steps 1 and 2. Try slowing the ramp rates and increasing the hold times. You may also want to sort your bottles. If you have thinner bottles that slump easier, put them in areas of the kiln where it takes longer for the heat to reach. If the heat is mainly provided by a lid element, and less so the sidewall element(s), put the thinner bottles on the lower shelves. This will help get you similar results on all of the bottles, no matter where they're located in the kiln.

• I see that some firing sequences have many more steps. Are those better?

  Maybe, but not necessarily. It depends on your kiln, and bottles, and what results you're looking for. For example, we have a much more lengthy sequence in our Tip #92. However it was developed by someone who was firing multiple layers in a much larger kiln, without lid elements. That's very different than firing a single layer in a Trio. Unless you have a better starting point, I recommend you start simple and go from there.

• Someone mentioned "flash cooling". What's that? Why would I want to do it?

  Flash cooling is also called "crash cooling" or "flash venting". The idea is to quickly take your kiln from its current temperature down to about 1100, to prevent the glass from continuing to slump. To do it, you crack open the lid of the kiln just as the bottle reaches the perfect slump. Once down to 1100, you close the lid and have the kiln run through an annealing step to take the stress out of the glass. (The annealing step is very important when you flash cool.) However flash cooling is stressful to your kiln brick, molds, heating elements, and potentially your glass bottle. It can stir up dust that may land on your bottle. I've never found it necessary when slumping bottles, but you may feel differently. If you decide to do it, please take proper safety precautions such as temporarily unplugging your kiln and wearing protective clothing. (You want to unplug your kiln so the heating elements are not energized while the lid is open. That's for your safety from electrical shock, and to reduce thermal shock to the elements from the cool room air.)

• I have a kiln with a KilnSitter. Can I slump bottles in this kiln?

  Yes, but it's very difficult to get good, consistent results. Firing without an electronic controller, or at least a pyrometer (for a temperature readout), is basically a shot-in-the-dark. If you want to try it, put an 017 cone in the KilnSitter and set your heat switches to a medium setting, You may find you need to turn the switches up towards high at the end of the firing. If the resulting bottles are not flattened enough, try again with an 016 cone. If too flat, try 018.

• I'm worried I'll never find a good firing sequence. What's a good way to develop one?

  Don't try to figure it all out at one time; Instead take a steady, incremental approach. Start with the generic firing sequence above (or some other starting point you like). Start a log to record the results of every firing. Record the firing steps, the results, how you loaded the kiln, and some general notes. I take pictures of my completed bottles for every new firing sequence I try. (You think you'll remember and keep track of this stuff in your head, but you likely won't. I can't. I refer to my firing log constantly.) Use some of the suggestions elsewhere in this tip to determine what changes to try, based on the results you're getting. Don't modify too many things at one time (limit it to 1, or 2 at the most), so you can figure out what caused the changes you'll see.

  Also, just because you have a big kiln does not mean you have to fire it full of bottles every time, particularly when it's new to you. Try loading just a few bottles and see what happens. Put in empty molds or extra kiln posts into the kiln to simulate objects that would be there had the kiln been full. If you have a two layer kiln, work out the process for one layer first, then add bottles to the other layer on subsequent firings. If you are trying to make apples to apples comparisons, be consistent about the style of bottles you use and where you place them in the kiln.

• Can you show me an example of a bottle that came out badly and how you adjusted the firing to get better results.

  Sure. A friend asked me to fire some cool-looking beer bottles. The first firing came out bad. The bottle had jaggy edges and the lettering got washed out. I trapped some air bubbles in there too.

Step         Rate                               Set                                          PtHold

1	500	1100	10
2	250	1300	0
3	300	1440	25
4	999	950	40

This bottle was fired too hot, and held too long. The result was a jaggy edged with washed-out lettering. The neck also collapsed too soon, thus the trapped air. I used a firing sequence that worked Ok for slumping a non-lettered bottle flat to the shelf. However it was a poor choice for a bottle with lettering in a curvy mold.

To fix the firing, I changes a few things (which goes against my recommendation of only changing one thing at one time, but I only had one more bottle to work with.) Fortunately, it worked out. I added a brief hold time to Step 2, in the hopes that it would help get rid of the trapped air (i.e. collapse the middle of the bottle before the neck). I reduced the temperature and the hold time in Step 3, to eliminate the jagged edges and keep the decal from burning out.

Step         Rate                               Set                                          PtHold

1	500	1100	10
2	250	1300	10
3	300	1425	15
4	999	950	40

A much nicer slump. No jaggy edges, no trapped air, and a nicely preserved brewers decal. This is a great example of how much difference a few adjustments to the firing sequence can make. I think the significant changes were those in Step 3, not as much the hold in step 2, but it likely helped some

We hope this tip helps you slump some great looking bottles. If you make something cool, send us a picture and your firing sequence. We'd love to see what you did and how you did it.

See our other Bottle Slumping Tips and check out our large selection of kilns:

• Overview

• Preparation