In a medium sized bowl, add in the Epsom salt, baking soda, corn starch, and citric acid. Then whisk the dry ingredients together. Mix well to break down any clumps.
Next, in the small jar with the lid, add the grapeseed oil, water, food coloring, and a few drops of essential oil. Place the lid on and shake well.
Remove the lid from the jar and slowly incorporate the liquid mixture into the dry. Slowly whisk the ingredients together. Your goal is for the ingredients to NOT start fizzing.
Once all of the liquid has been added, continue stirring until you have the color consistent throughout all of the dry mixture.
Next, fill the molds with the mixture. Make sure to stuff each side of the mold completely full.
Press both sides of the mold together and let them set. It's best to let the molds set for a minimum of 24 hours.
Once the 24 hours is over, place the bath bombs in an airtight container or in Ziploc bags.