Flattened glass bottles can make interesting pieces of art, themed drink trays, or cute cutting boards. It's impossible to "melt" bottles with common household tools, but if you have a stove, the process will be easier and more fun to try. Remember, if you have an accident while working on glass, make sure you call emergency services immediately.
Part 1 of 3: Preparing for Furnace Installation
Step 1. Ensure access to the stove
The glass must be heated to 815ºC for the glass bottle to deform. To reach this temperature, you either have to find a ceramics maker that rents out the stove, or buy an electric one.
Electric furnaces often require a new electrical circuit, which is installed by an electrician. Furnaces installed at the wrong voltage may fail to reach the correct temperature
Step 2. Follow the safety instructions
When working near a furnace, wear furnace gloves and safety goggles to protect yourself. Wear a breathing mask whenever you are working with a furnace or removing dust and debris from a furnace, and always operate the furnace in a well-ventilated area. Keep in mind that the inside of a stove can be much, much hotter than a toaster or fireplace. Before you begin, read the stove's instructions for use, or ask an experienced tiler or glassmaker for advice.
Step 3. Protect the furnace floor and racks
If you skip this step, shards of broken glass can damage the floor and stove racks during combustion. There are three common ingredients that can be used to prevent this, and all of them should be done wearing a breathing mask. This protection should be re-applied when it starts to look uneven, peeling, or crumbling.
- A glass separator (recommended) or furnace cleaner (allowed) can be purchased in powder form and mixed into a liquid. Apply at least four coats, then wait for it to dry. Make the surface even, because a little irregularity can be seen in the glass results.
- Alternatively, cut a sheet of parchment paper (fiber paper) to the shape and size of the rack. Place it in the oven and burn it to 760ºC to darken the paper, which can then be used as a shield between the glass and the shelf.
Step 4. Insert the rack into the furnace
The furnace rack should be above the floor of the furnace, to allow air to flow between. Place the ceramic kiln support on the floor of the kiln, then place a shelf on top of it. When you are about to start burning, your bottles will be placed on the rack.
Part 2 of 3: Preparing the Bottles
Step 1. Make a ceramic mold (optional)
If you prefer your bottle to curl like a taco shell instead of a flat tray, press your bottle against the clay to make a mold. All molds must be protected with a furnace cleaner or glass separator, as described in the furnace preparation section.
Use clay intended for combustion at 815ºC, or the clay may melt during combustion
Step 2. Clean the bottle and remove the label
Rub the bottle with hot, soapy water, or leave it in a bucket of hot water and laundry soap for a few hours. Rub all paper labels and stickers clean, or scrape them off using a hard plastic object. Alternatively, if you want to save and re-attach the paper label, melt the adhesive with hot glue.
- Painted labels will survive the burning process, and can be a good design if the bottle doesn't move during combustion.
- To prevent fingerprints from sticking, put on gloves and clean afterward with isopropyl alcohol.
Step 3. Apply a devitrifying spray (optional)
Also known as "devit," this product actually prevents devitrification, or crystallization of the glass that causes it to appear foggy. Not all types of glass are susceptible to devitrification, and cleaning the glass can be very helpful. Use the spray if you want to be very careful, especially with the blue and brown bottles.
Step 4. Add hanging wire (optional)
If you want to hang your bottle that has been flattened, form a piece of wire into a hook and thread the other end into the neck of the bottle. The bottle will melt tightly around the wire, so you don't have to install it yourself.
Heat resistant wire is the best choice. Most wires will work, but aluminum can melt, and copper and brass can leave spots on the bottle
Step 5. Keep the bottle from rolling
Place the bottle or bottles placed on top of the mold on the stove rack, horizontally. If there is a risk of rolling, hold it down using (the same) broken glass or small rolls of parchment paper. This will leave a mark on the back of the bottle, but is better than the bottle rolling sideways and damaging the walls of your furnace.
Take care to keep bottles with painted labels moving
Part 3 of 3: Flattening a Glass Bottle
Step 1. Heat the furnace to 590ºC
Heat the furnace to a temperature range of +275ºC per hour, until it reaches 590ºC. This will just start heating the bottle.
If you are using a ceramic mold, you will probably want to use a slower heating speed to reduce the risk of breaking the mold
Step 2. Hold this temperature for ten minutes
“Soaking” the glass at this temperature ensures that every piece of glass is at the right temperature. Follow the steps below to find out how long you should hold the furnace at a certain temperature.
Step 3. Heat more slowly to 700ºC
This time, preheat the furnace at a rate of no more than +140ºC per hour, for a little over an hour. At this point, the glass will start to deform especially in the middle. You can hold this temperature for 20 minutes if you want the center to be flatter and wider, or continue after a few minutes if you want the center to retain its shape a bit.
Step 4. Heat quickly to 790ºC
Heat at +165ºC) per hour if you're using a ceramic mold, or faster if you're not. Hold at this temperature until the bottle has melted to the look you want.
- This is the step that changes the most, based on the bottles, furnace, and appearance of your desired result. Consider these numbers as a starting point for your first project.
- Always wear eye protection when you peek into the peephole. If your stove doesn't have a window or peephole, you won't be able to check your bottles.
Step 5. Remove the air in the furnace until it reaches a temperature of 540ºC
Lift the lid of the furnace – taking care to protect yourself from the heat – to cool the furnace quickly to temperatures between 480 to 590ºC. The less time the bottle spends at high temperatures, the lower the risk of devitrification, or the formation of a misty surface texture.
Step 6. Glow the glass
Glass undergoes significant stress when heated, and can crack or become brittle if not "ignited", a process that rearranges the glass molecules into a more stable pattern before cooling. There are two commonly used ways to do this:
- The simplest way, which is usually sufficient for bottles, is to let the furnace cool slowly, never more than -80ºC per hour. If your furnace is cooling faster than this, you will have to re-burn it every now and then to counter the cooling.
- For a more effective igniting, leave the furnace at 480ºC for a full hour. Different types of glass have different optimal tempering temperatures, so to be on the safe side you can leave them at 540ºC and/or 425ºC for an hour at each temperature, starting with the highest temperature first.
Step 7. Allow the furnace to cool to room temperature
The bottles should have melted flat. If you are using parchment paper and the lint is stuck to the bottle, wear a breathing mask when cleaning it from the glass.
- If you've removed the paper label and are planning to re-attach it, try sticking it on the bottom side of the bottle to get a neat look, and protect it from damage.
- Make a note of each process you use each time. A little experimentation will determine the best process for your furnace and bottles.