Introduction: Lightbulb Knife Sharpener

Picture of Lightbulb Knife Sharpener

Sometimes you really need to sharpen a knife. But what if all you have on hand is a burned out high pressure sodium light bulb and desperation? You're in luck! The bulb's quartz core can be used to sharpen knives, tools, and your reputation as a genius. If you've got a couple basic tools and a dowel, you can upgrade that quartz to a nice sharpener that's also a great conversation starter.


High pressure sodium bulb (burned out, unless you want a sharpener more than a working bulb)

Dowel, 1" - 2" diameter, whatever feels comfortable

Washer, same diameter as the dowel


String, flammable but slow burning


Drill press or drill

Nice but not strictly necessary:

Glass cutter


Step 1: Break the Bulb

Picture of Break the Bulb

You have to be careful not to break the quartz core when you do this, so I decided against smashing the bulb on the ground. I tried a couple other methods and realized the bulb was much tougher than I thought. Eventually, the combination of scoring, burning, and tapping did the trick.

It helps to score a line around the bulb to guide the break. I set up a board against the dogs on my bench, clamped in a glass cutter, and braced the bulb against the board to get a straight line all the way around. I spun the bulb several times to get a nice cut.

I then tied a string around the scored line and set the string on fire. You want a string that will burn for a little while to get the glass nice and hot on that line. When the string burns off, immediately dunk the bulb in cold water. You should hear a crack from the cooling glass pulling apart.

If your bulb didn't break in half in the water, tap it gently on the edge of something. You should end up with two halves of a bulb and an exposed quartz core.

Step 2: Remove the Core

Picture of Remove the Core

Sometimes you can just wiggle the core out once you've exposed it. If it is being stubborn, gently break off the rest of the glass so you can reach the bottom of the quartz and pull or cut it out. This is the part that makes this broken bulb into a really great knife sharpener.

Step 3: Make a Handle

Picture of Make a Handle

Cut a dowel to the length you want your handle. You can either sand down the dowel and call it good or, if you have a lathe and a handy turning tool (see my instructable for a turning tool here), you can make a grip and some embellishments. When your handle is the shape you want it, sand and stain it.

Drill a hole in the top of the handle just barely bigger than the diameter of your quartz.

Step 4: Put It All Together

Picture of Put It All Together

Use epoxy to secure your quartz in the handle and glue a washer on the top of the handle.

And you're done! You've got a quality knife sharpener made on the cheap from a burned out bulb and a dowel. Time to make all of your knives much sharper.


mthomp1 (author)2016-04-19

The quartz core (arc tube) of the high pressure sodium lamp is not made of quartz. It is actually made of ceramic (poly crystalline alumina). It is quite hard. The outer bulb is made of borosilicate glass which is also hard.

Keep in mind the space between the outer bulb and the ceramic core (arc tube) is vacuum. The bulb will implode if dropped to the hard floor. The safe way to relieve the vacuum is to pry off the center button of the lamp base. Poke a nail through that hole into the glass stem and tap on the nail to break it. Once that seal is broken, you will hear air rushing into the bulb. The arc tube will remain intact.

Then you can cut the bulb apart safely.

Yonatan24 (author)2015-12-14

Hi, I've added your project to the "Unusual Uses for Light-Bulbs" Collection

This is the link If you are interested:

prathiths (author)2015-04-01

you have not given sufficient importance to the safety issue while breaking the bulb,which releases sodium vapours,can also explode!please elaborate this in detail,and explain how to break without endangering oneself to the risks.

BSGMutha (author)prathiths2015-04-24

people need to educate themselves also. this is a nice guide. it isnt this writers job to hold everyones hand.

theskashow (author)prathiths2015-04-01

Here is a link to the MSDS for these bulbs for the health risks. Shouldn't be any worry about exploding but you are right, making sure to take precautions with the broken glass is always a good idea. Gloves and glasses are never a bad idea.

GudRabbit (author)2015-04-06

My pops showed me those things not to long ago.

Joro 05 (author)2015-04-04

At first - bulb can be opened more safely by once sharp hit near to socket with big iron nail or key file. Usage of safe glass, gloves or/and plastic bag is desirable,especially for first time.

Second - I wouldn't use the torch (internal bulb) for this purpose. Actually It appropriate for this, because it is made from AlO3 (sapphire or corundum) but it still works. I would use it for lamp. It will glow when you give it HV by some toys (HV power supply in this site)

Corinbw (author)2015-04-03

so where do you get these high pressure sodium lights

theskashow (author)Corinbw2015-04-04

Your best bet is to talk to someone who works in electrical or maintenance and see if the will save you one. If you tell the what its for and maybe offer to make a second for them that should do the trick.

cruelgreedy (author)2015-03-31

I read this yesterday on a whim. Tonight on my way to work I looked in a trash can, purely by chance, and found this in it... apparently it's destiny for me to have one.

theskashow (author)cruelgreedy2015-04-02

That is one lucky find, very nice.

Is that so...?

prathiths (author)2015-04-02


eckyeckypikang (author)2015-03-31

I read this yesterday on a whim. Tonight on my way to work I looked in a trash can, purely by chance, and found this in it... apparently it's destiny for me to have one.

BTW... according to a few sites I looked at the core is made of a ceramic called aluminum oxide (I am not a chemist...) and is renowned for its strength, hardness and abrasiveness. It's apparently commonly used in making sandpaper. Perfect for this application!

Thank you for your timely 'ible! Had you not pushed it, I'd have walked right by this thing without a second thought.

Crankin01 (author)2015-03-31

Thank you for sharing, I has been using these for years to sharpen my knives. I got mine when from the light bulbs we changed on my college campus when I was a student worker. Since these aren't common for home use, what are other sources where you could find these discarded bulbs?

theskashow (author)Crankin012015-03-31

I work around electricians and most of them would be more than happy to save one if they got asked. I would guess the same would go for maintenance people as well, just ask around.

bakichmg (author)2015-03-30

Good idea. Please be careful of mercury exposure.

theskashow (author)bakichmg2015-03-30

Good call, the MSDS on these lamps says " No adverse effects are expected
from occasional exposure to broken lamps". Still a good idea to keep the area ventilated and don't make to many.

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