I think this is a fairly simple project for those of you that want to get your feet wet casting concrete. This was the second cast concrete project that I ever tried and I think it turned out pretty good. There are a few nuances that make it a little more advanced than making a stepping stone or a coaster, but fear not, you have an Instructable and soon you can have a really cool custom lamp!
Step 1: Round Up Your Materials
Here is a list of materials needed:
A form for your lamp base ( I used a Highway Cone for this one)
Maximizer Concrete Mix or whatever your local store carries
Clear packing tape
A container or two for mixing concrete
Implement for mixing (a hoe or small spade works great)
Rubber gloves (if you're messy)
Mineral Oil, Non-stick cooking spray, baby oil, etc. to aide in removing the form
Lamp Wiring Kit (or scavenge parts from an old lamp)
Lamp Shade (I used a small $2 trash can purchased at Target)
A thrift shop is a great place to go shopping for this project. The local Goodwill Store had several lamps that would have been great for parts ($2 each) and all sorts of glass and plastic vases that could be used for forms (under $2). You can use a glass vessel for a form, but they call for tools and techniques that will not be covered in this Instructable. They work great when the shape is one that wouldn't ordinarily allow you to slide the mold off. With a glass form, after your cement has hardened, you can carefully break the glass in order to de-mold the piece.
Another money saving tip for those on a budget, is to inquire about damaged bags of concrete mix at your local home improvement store. They are notorious for getting torn and spilling. You don't need a full bag for this project and you might just get a damaged bag for the bargain price of free ninety free!.
Step 2: Prepare Your Mold
The first step in this process is preparing your mold. Take your time. Make sure whatever you are using for your form is smooth and clean.
Step 3: Affix Threaded Rod and Run Cord
Since I am using a cone that is open on both ends, I had to close off the small end and leave an opening just large enough for the threaded tube that the lamp wiring will pass through. I used a small metal disc with a hole drilled in the center. Using silicone caulk, I attached the metal disc and sealed up the small end of the cone. If you use something like a vase or other vessel with a solid bottom, you can just drill a hole for the threaded rod.
You have to plan for your "plumbing" so to speak. You need to have the threaded tube that comes in your lamp kit attached inside of your form and the wire already running through it. You can fabricate a 90 degree elbow and have the tubing totally encase the cord or you can just use a small section of tube for where the cord enters and exits the lamp base. If you use two small sections, like I did here, put a dab of silicone in the inside of each interior section to keep water from the mix from leaking out. I made a small cut out in the cone to allow the wire to come out of the lamp base an inch or so above the base of the lamp. You could have it come straight out of the bottom, but you would have to attach feet of some sort to the base, in order to raise it up above the cord so that it would sit level. I attached a nut on both sides of the metal disc plugging the small end of the cone and I have a nut on each side of the cone where the cord will come out at the bottom. I used clear packing tape attached to the inside of the cone to cover the part of the notched out area that was left open. The tape is strong enough to hold the cement mix in, but still allow me to slide the form off later. After attaching the tape, apply a light coat of oil to the inside of your form.
Step 4: Mix Your Cement Mixture
Make sure that you mix enough cement mix to fill whatever form you are using. Better to mix too much than not have enough! Add water a little at a time and mix thoroughly. You want the mix to flow enough to fill the form, but not be "soupy" . I mixed two batches of mix. One is colored with Charcoal Quickrete liquid color and the other is straight from the sack with no color. If you chose to go for this effect, it is important that your mix is fairly firm (softer than peanut butter but firmer than I like my oatmeal) and both batches need to be this consistency. If they are soupy the colors will bleed together and you wont have the contrast between the two colors.
Step 5: Filling the Form
If you are just using one color fill the form about 25% full and lightly tap the sides of your form to remove any air bubbles, then fill the form half full and do the same. Repeat until you have filled the form completely. If you are using more than one color, use the same procedure, but place the colors into the form in whatever way will create the effect you are looking for. I randomly placed mine to get a swirl type effect. Be careful when adding cement mix and make sure your metal tubing is staying put. Once the form is full, make sure it is secure and level. Use a trowel or straight board to smooth out the cement mix level with the top of your form. Place a piece of plastic over the top and keep the form in a shady location. The concrete needs to dry slowly in order to be strong. The hardest part of the whole project is waiting 2 days for it to dry!
Step 6: De-Molding (The Fun Part)
After two days drying time you should be able to de-mold your lamp base. Work over a padded surface of some sort and gently coerce the form off.
Step 7: Finish Wiring, Install Hardware and Shade
Finish wiring your lamp according to the instructions that came with your kit. Install the harp (the part that holds the shade), the correct wattage bulb and then the shade itself. I drilled a hole in the bottom of a plastic trash can and used it for my shade. I placed a rubber washer between the shade and top nut. I plan to use this lamp on my deck and the totally enclosed plastic shade will keep the water out. It looks great indoors or out though. Enjoy your custom cast concrete lamp.