The major asset of latex molds is their ability to produce candles in a huge variety of shapes and a great precision in the details. This is something a metal or polycarbonate mold just can't offer.
And what makes them really attractive too is that they're easy to make yourself with very limited supplies, available in almost any DIY superstore.
Your first task is to find an item to cast and - very important - that is suitable to create a latex mold: unfortunately, not every item will do the job.
To give you an idea, the selected object must have a shape that is more or less «block-like». But if you don't intend to actually light up the candle you will create in the mold you're about to make, then you're totally free in your choice.
Another point is that the object cannot have any open part (for example, a character with the hands on his hips, because of the open space between the arms and the torso). Obviously, in such a case, it would prove impossible to separate the finished mold from the model. However, if there's only a very small open space, you can always fill it with some play dough or mold sealer.
Try and avoid items that have been painted, unless they've also been varnished. I once tried to cast a latex mold from a small resin gargoyle; when it was time to remove the mold from the gargoyle, a large part of the stone-colored paint stuck on the latex: gargoyle beyond repair, mold ruined...
For this project, I went with a Marsupilami-shaped salt shaker, ideal because the «body» of the salt shaker is shaped like a small pillar candle. The resulting candle would burn quite well.
To create the mold, a good option is liquid latex. You can find this kind of latex almost everywhere: in craft stores and in most DIY superstores.
Make sure the room you work in is well ventilated: liquid latex contains in average 40 to 50% ammonia (to help keep the latex in a liquid state in the bottle). Ammonia is not the most pleasant product to work with and is far from healthy to breathe in. Always try, even in a well ventilated room, to keep mouth and nose below the level of your paintbrush and do not inhale the emanations (rest assured, after 5 minutes with your nose above the bottle of latex, you will see Marsupilami's everywhere around you!)
Shake the bottle thoroughly before you start, otherwise the latex on top of the bottle (right where you dip your paintbrush) will be way too liquid to be correctly and efficiently applied. Don't forget to close the bottle when you're done with a layer.
Every brand of latex is different, so always follow the instructions of the manufacturer: how to work with the latex, minimum number of layers and how you clean the paintbrush can vary from brand to brand.
Place the object you want to cast on a sheet of waxed paper.
Apply the first layer uniformly on the entire model. You will notice that the latex «holds» better on some parts of the model than others. In this example, I had to insist a lot on the ears and nose of the Marsupilami because gravity made the latex flow downwards, leaving too thin a layer on top. Do not hesitate to add more latex on these delicate spots and don't forget to regularly shake the bottle to make the latex thicker. But don't exaggerate: if you shake too hard, you will create air bubbles in the liquid latex. These bubbles should absolutely not end up on the mold that is taking shape.
It is important to create a sort of «lip» at least 5 mm wide at the base of the model, or in other words on the waxed paper. This lip is necessary for 2 reasons: to strengthen the open side of the mold, and to allow the mold to be hanged on a cardboard plate (see the article about flexible molds). The lip will also help unmold the finished candles. On picture 4, the bright white lip is clearly visible at the base of the model.
Depending on the brand of liquid latex you use, the drying time between each layer may differ. Always wait at least one hour (two hours is better, some sources say four) for the previous layer to dry before you start applying the next one.
«Fresh» latex is milky white and turns yellow as it dries. Always wait until every white spot disappears before you move on to the next layer. Some places where more latex concentrate, like the lip, need more time to dry than others.
Your mold will need between 10 and 15 layers (here again, some sources mention 20 layers) to be thick and strong enough to bear the heat of the paraffin you'll pour into it. This picture shows the mold after the sixth layer of latex has been applied.
When the last layer has been applied and the latex is entirely dry (to be on the safe side, wait at least 24 hours after you apply the last layer), remove the waxed paper (be careful not to shred the paper or you will have a hard time removing every little bit of paper from the mold base).
Put a little dishwashing liquid on your fingers and rub the whole surface of the mold with it: this will prevent the latex from sticking to itself when you will unmold the model.
As illustrated in picture 5, gently pull the lip upwards and remove the mold from the model like you would remove a glove (therefore, latex molds are commonly called "glove molds"): it will end up inside out.
Your latex mold is finished and ready to use.
Do not expect it to last forever: you should be able to cast an average of fifteen candles in the same mold before it starts to get bristle and deteriorate... at which point you will have the pleasure to make a brand new one.
NEVER add stearin to the paraffin when you use a latex mold. Don't forget that stearin is an acid and that it would eat at the latex without mercy.
To further extend the life of your latex molds, store them in a dark place and, before you unmold a candle, always rub some dishwashing liquid on the outside of the mold.