Instructables

Step 3: Mix up the Concrete

This is the part that's more art than science. I've found that in the small batches this project calls for, it's very easy to add too much or too little water to the mix. When you are mixing an entire sixty pound bag of concrete, being off a teaspoon on the water doesn't matter that much. When you are mixing up just a cup of concrete, that teaspoon starts to matter.

Mortar mix when cured in a glass mold like we are using gives a very nice gloss surface. The lower the amount of water you use, the smoother and like glass the surface is, and the stronger the resulting cured concrete is. However, the lower the amount of water you use, the harder it is to have it fill in the gaps on the sides and it leaves lots of holes and divots. It's getting the mix liquid enough to spread out but not too liquid that it looses it's strength is one of the issues on the water to dry mix ratio. Finding the correct compromise between these two issues is really a matter of practice and personal taste, I would suggest you play around with it in multiple bulbs if you are interested in getting the best result you can.

The ratio I've found works well is about 1.25 cups of the mortar mix and a hair under four tablespoons of water. So measure out a little under four tablespoons of water and put that into your plastic tub. This is more mortar mix than you need to fill a lightbulb, but there is always some spillage and trying to reduce the amount means even more accuracy on the water measurement. This is a good place to start.

Slowly mix in the mortar mix a little at a time. Let a little bit get wet, then a little more, then a little more while stiring the hole thing. It's a bit like making biscuit dough at this level, but your pouring the dry into the wet instead of the other way around. The consistency should be good enough that the mortar mix wants to stick together in one large clump, but it isn't sopping wet. If you feel you need to add more water or mortar mix to get it correct, then go for it. Just do it a little bit at a time. A small amount of either material make a large impact at this point.

Once it's at a consistency you like, keep stirring nice and slow for a few minutes. You want everything to be throughly wetted as much as possible.

Important! At this moment the clock starts and you have 30 to 45 minutes to finish the lightbulb before the mortar mix starts to harden up.

Once you have throughly mixed up the mortar mix, bring out the lightbulb and start filling it up.
 
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jumper173 years ago
That caption on the last picture of this step threw me off. I have a tendancy of not finishing what I start, so I went to my oven and set the pre-heat to 375. Then, after setting the timer as well, I came back and finished reading the caption. I did my first ever, REAL facepalm today.
What effect would substituting the mortar mix with tile grout, as we have some left over from re-doing the floor. would this crack up easily, have a negative effect on sitting in large amounts, or wont work altogether. -thanks
whamodyne (author)  binglebeebop6 years ago
Tile Grout does well in the lightbulb. Be aware, it's more "sticky" than mortar so it likes to stick to the rim of the bulb and your fingers more than regular mortar would. It's not as sticky as the thin set mortar you use for tile (which was an ungodly mess the couple times I tried it) but still more sticky than mortar. I'm using a pure white grout as a base for most of my colored bulbs these days, it's cheaper and easier to find than a white mortar mix. Using a white base and adding pigment was the best way I found to get a consistent, bright color. Grout can also be mixed up wetter than mortar and not have the massive cavities and fall apart like the mortar would. However, the look is usually a bit more uniform than the mortar which has these wonderful color variations on the bulb, almost like granite. Mortar is also still the king of all the mixes I've tried for the shine you get straight out of the bulb. Grout will reflect well and give you a smooth surface, but doesn't have the glassy, polished look that mortar does.
if you do the tile grout bulb, and it doesnt turn out glossy like mortar would, could you spray several coats of varnish on it to get it glossy?
Of course, But do it lightly because Stalactites will form if you spray heavily. Also, don't set it down (Suspending could be a problem).
Jouda Mann6 years ago
About the bubbles: Since you're working with a small piece, I would suggest getting a palm sander. Five bucks at any pawn shop, and it doesn't matter what condition it's in, as long as the motor runs. It works by rotating the head in very small circles, and vibrates nicely. Set it in a jig upside down, and turn it on when you need it. It has a dense foam pad on it, so it will cushion your work as well.