It can be so frustrating with bath bombs when you are first starting out. With time (and many failed attempts of batch after batch) there are some handy things to look for in a failed bomb that might be clues as to what went wrong.
A solution to try - These are formed by too much moisture either in your recipe or humidity in the air. Try cutting back on your water content especially during humid and rainy days.
A solution to try - When your bath bombs are expanding or activating after you’ve made them it is usually due to too much moisture. Try to cut back on how much water or other binders you used next time. Humidity also plays a big factor in this, especially when it’s raining. Some people also use salts in their recipe and sometimes salts draw in moisture from the air and can make the bath bombs activate.
A solution to try - Cracks and lines can happen for many different reasons. Using clay in your recipe can dry out the bath bombs too fast sucking up all the moisture causing cracks. Another reason could be that you’re not using enough oil in your recipe and they’re more brittle from this. Cracks that usually form around the centre of your bombs and make a split are from packing too tightly. As the bath bombs are drying there is a slight activation that is happening and when packed too tightly the mixture grows slightly and will push apart. another reason cracks can happen is from using embeds inside that are activating. This can happen if you’re using embeds that aren’t fully dry or you bath bomb mixture is slightly too wet. That moisture can cause the embeds to activate and expand creating cracks from the inside out.
Cracks or spitting along joint lines are indicative of hard packing to the point that the mix can’t adhere together.
A solution to try - Try packing less into the two halves before squeezing together
A solution to try - Usually from using too much oil or not enough binder to form the slight activation that is needed for the bombs to hold together. It could also be from too little oil. The bath bomb will be powdery and crumbly.
A solution to try - Let bath bombs dry in something with a rounded bottom. Those clear bath bomb moulds usually called Saturn Ring moulds are best for keeping your bath bombs in a nice round shape.
A solution to try - Speckling colours usually occur when the colours have not been properly mixed in your recipe. Make sure you mix very well before moulding you’re bath bombs. Using dyes require a lot of mixing because they are so concentrated. The more you mix the more the colour will develop. Make sure when diluting you’re colours you mix them well in your liquids and then mix well again in your recipe.