I saw a tutorial several years ago about filling light bulbs with cement. The tutorial uses a long screw put into the in the cement at the end of the lightbulb and screwed the concrete bulbs into a board so you could use the bulbs as a hook. I liked the idea but I wanted to do something else with the finished product.First you need to empty the bulbs and there are several tutorial on how to open & empty a light bulb. It's really easy but as always wear protective eye wear and gloves so you do not cut yourself. I then place the bulbs in a paper cup with sand on the bottom and a little on the top so the bulb stays up right and will stay in place when I start to fill then with the cement mixture. I use a ratio of 1 to 1 of play sand and portland cement. I use play sand because it's fairly smooth, thought if you want to go cheap you can use construction sand. I would sift the construction sand through an old wire strainer and get the larger pieces of rock out of the sand. You would be surprised how much a small rock can mess you up when trying to get the cement into the bulb. I find out how much cement I need to mix by first fill the light bulbs with sand and measuring that amount. I always make a little extra cement as not all the cement always makes it into the bulb. I mix the cement to a cake batter constancy.
I fill a plastic bag with cement and cut a hole in one of the corners, using the bag like a pastry bag and filling the light bulb with cement. Some folks use a funnel but I have had problems with the funnel clogging, but that does not mean you should not try it..maybe I haven't found the perfect funnel yet. You do have to work quickly because cement hardens. I can make about 4 bulbs at a time before I have problems with clogging of the plastic bag.You also have to do some tapping to get all the air bubbles out and get the cement settled into the light bulb. This is where having the light bulb in a bed of sand helps to keep the bulb from breaking. I have broken a couple of bulbs by tapping the bottom with my hand to get air bubbles out.
Once the bulbs are full I slowly put a piece of rebar into the hole in the light bulb. Cement will come out on the sides but don't worry about it you can clean up at the end. I let the bulbs sit for about a week. Yes this is a long time and I get excited and want to get moving but there is nothing worse then trying to clean the piece or take off the glass only to have the cement crack or fall apart in your hand. All that work for nothing. During the summer time when the weather is warm the water will evaporate more and the cement does harden a little quicker, so you may be able to clean it and break the glass off in about 4 days.
These are larger bulbs where you can see the cement pouring out the sides. I try to put at least a 4ft piece of rebar in the bulbs. That way you can use them to hold up plants. The light bulb on the left was a flood light bulb. I want to mosaic the top then grout the piece with glow in the dark grout...kind of like this. I have used a product made by Laticrete called Spectorlock Dazzle. I used it on one of my tiki head...the head shinned but it never really glowed in the dark. That was also 5 years ago; so maybe they have improved the product.
A variety of different size light bulbs looks great in a garden. Last year my concrete ideas in my community garden. I used wire fencing held in place with my concrete ideas on each end. I grew my tomatoes up the fencing. The concrete idea is a good thing.