I am assuming that you are measuring 12 mL in a flask. This is very unusual because Sulfate is in a solid form therefore you are best to weight the sample in a balance on order to determine the solubility component to hold in the water volume.
The density of Alum assumed to be the unhydrate is 2.572 grams per mL therefore you have 32.064 g of the anhydrous form. The solubility i water if 31 to 36 g per 100 mL. You are adding in 100 Ml of water in laboratory analytical conditions. Therefore, you are closing too close to the solubility lists at room temperature.
Another effect to consider is the dramatic increase in specific gravity. The water volume in the solution will shrink to compensate the final specific gravity of 1.33. This will saturate the solution and will precipitate to accommodate specific gravity and normal solution solubility at the temperature.
This chart will help you to evaluate how the ratios of sulfate weight and water volume impacts the specific density the Aluminum Sulfate solution.