Hey family,
I’m creating symbolism in my room which includes the elements of mercury, sulfur and salt.
The only place I’ve been able to find mercury is in old mercury glass thermometers.
I plan on breaking the glass and storing the mercury safely in a special seal jar that fits in one of my small bowls.
Question though is for the for elements. Air, water, earth, fire. Can I use the broken bits of glass to represent the earth element? Maybe I’ll also add some soil from my local area with it.
Tem
I would think that glass would also be a representative of fire, given that it’s sand that’s been melted to ultra-hot temperatures.
You’re right. And given same glass comes from a thermometer to measure temperature, perhaps I will use it to represent fire.
Ohhhh this is fun- there are so many ways you could go about representing the elements! 
I absolutely agree with Amethyst that the glass could represent both earth (sand) and fire (fire needed to form the glass).
I think you will find that you have lots of options with finding things that represent multiple elements (it’s a fun activity to look around and examine what things contain which elements) 
As most items have been created from using different elements, I actually think it’s harder to represent pure forms of each element.
For representing elements, here are few additional options to consider:
ALL: Symbols or Sigils- although not necessarily made of the element itself, a magickal symbol or sigil is an easy and neat way to draw on the power of each element
ALL: Crystals- crystals are formed over many years by each of the elements: earth shaped by heat, pressure, and other natural influences. Additionally, every crystal has elemental correspondences.
Earth: rocks, crystals, dirt, plants, wood
As earth is the most stable element, earth items will likely be the easiest to find and maintain
Water: water (kept in a cleansed bowl or vessel), ice, plants (made up of mostly water), sea glass, river stones, sea shells
Air/Wind: air, feathers, empty space, dried leaves
Wind/Air can be the hardest or easiest element depending on how you go about it, as air is a constant presence- always there but hard to contain
Fire: candles, lava rocks, glass, charcoal, ash
While traces of fire energy are easy to come by, keeping the element itself can be difficult and dangerous
Just a few suggestions! I’m sure whatever you choose to represent the elements will be perfect. Good luck and have fun developing your sacred space, Tem! 
You’re very welcome, Tem- I’m happy if I could help! Good luck and blessed be 
