How To Grow Crystals
To grow crystals at home, you don't need to weight out anything or measure anything. Other than ordering a compound online, you'll just need to grab a few supplies from your local grocery store:
clear plastic or glass cups
thin fishing wire or string
Method 1: To grow large crystals over the course of 1-? months.
Step 1. Choose a compound to work with. As a general rule, you want a salt that is non-toxic/not hazardous and readily soluble at room temperature. Common choices for beginners are: K-alum, borax, sugar, table salt.
Step 2. Grow Crystals. Warm up some water by either running the tap until the water coming out is hot to touch, briefly microwaving some water, or heating it on the stove.
You don't want it to boil, because the greater the temperature difference between the solution and room temperature the more supersaturated it will be and you will rapidly grow many small crystals. A solution with a lower degree of supersaturation will grow fewer crystals more slowly, and they will likely be larger than those formed from a more supersaturated solution. Tepid water is best.
Once the water is warm, add a tablespoon of the compound to solution and stir until it dissolves completely. Continue to add tablespoons of the compound to the solution until the particles do not dissolve anymore. Note that as the solution approaches saturation, the dissolution rate of the compound will decrease so you will have to stir for longer amounts of time as you go.
Once I think the compound is no longer dissolving, I tend to add a bit more and to let it sit overnight, but that is not necessary.
When the compound is no longer dissolving in the solution, pour the liquid into a clean, clear container (I just use pint glasses, but shallower containers will be better/evaporate more quickly). Be sure to rinse the container with water to remove any dust or particles. Use coffee filters to be sure that none of the undissolved material goes into the new container.
To grow very large crystals very slowly (optional because it adds ~1 month of time to the process) : slightly undersaturate the solution by adding a dollop of water to it. This will ensure the dissolution of any nanocrystals or incipient nucleii that may be suspended in solution. Also this is the solution if you suddenly develop many small crystals instead of one big one - just undersaturate the solution to dissolve the small ones away then allow the solution to re-saturate again via evaporation at room temperature.
Use a new coffee filter to cover the solution. I prefer to use a rubber band to secure the coffee filter cover.
Some people like to put add nucleation sites to the supersaturated solution after filtering the solution into the new container. This will allow for crystals to grow on whatever particles or substrate you add to them. Do this if you feel like it, but if you are trying to make high-quality seed crystals to grow very large crystals with I suggest not. If you would like faster results and want to use a seed crystal, skip ahead to the "Method 2" section below.
Set the covered container in a warm, bright spot. I put them on my windowsill. Walk away and resist the temptation to repeatedly jostle/touch the container. Leave it alone. Check it about one a week. If you are diligent about not agitating the solution (which increases nucleation and so can result in many small crystals instead of a few large ones), after a month or so you should see some crystals!
Method 2 Grow a crystal in days or hours using a seed
Step 1. Choose Compound
Step 2. Grow seed crystals. Follow the same steps as above to create a saturated solution, but DO NOT add a dollop of water to undersaturate it a bit. Instead, filter it and cover it and leave it somewhere warm overnight. Again, do not touch it. Depending on the compound you selected you will have small crystals the next day or week.
Step 3. Grow a large crystal. Filter off the remaining liquid (called the "mother liquor" by romantic chemists) by pouring it through a coffee filter again into a new clean container. Remember to rinse the vessel you are going to grow the crystal in. This solution is no longer supersaturated, but almost exactly saturated.
Using a pair of tweezers or forceps, remove the best seed crystals from the container. Your criterion for "best" will likely be a mixture of size and clarity. Aim for the sweet spot of large enough to be handled easily with tweezers/forceps and clear enough to be seen through readily.
Using a very thin string or a piece of your hair, tie a cradle around the seed crystal you chose and attach the other end of the string to a toothpick or pencil. You will then suspend the crystal in the saturated mother liquor solution, cover it with a new coffee filter and rubber band, and leave it on the window sill. If you are okay with your crystal growing on the bottom of the container (as I usually am), then skip the cradle/suspension part and just drop a seed crystal or two into the mother liquor.
Leave the solution for a few weeks. Give it a few days between jostling it. Your crystal will grow slowly and steadily as the solution evaporates. As water evaporates, the compound becomes supersaturated in the solution and will grow new layers on the seed crystal.
Sometimes you will grow many small crystals that are not on your seed. When this happens the best thing to do is to re-filter the water and move the solution + seed (- small unwanted crystals) to a new container. Add a dollop of water and hope it does not happen again. Some compounds are more prone to this phenomenon of secondary nucleation than others. You do not want to grow the seed with a bunch of smaller ones because: 1) growth will happen much more slowly on the seed and 2) you risk the development of "parasitic" crystals (an unfair term for cute little crystals growing on the seed - I actually like them but many people think they are ugly).
Step 4. Wait. The best thing to do now is to forget you are growing crystals. The longer you wait , the bigger your crystal will be - as long as the seed is bathed in the solution.