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Lipstick hardness helps provide some insight as to the stability of lipstick for throughout the product lifecycle, from manufacturing to transportation, use and eventual end of use [55]. Hardness can be measured according to two common ASTM methods: ASTM D-1321 and ASTM D-937. These methods utilize needle penetration and cone penetration, respectively, in order to calculate formulation firmness in a quantifiable measure [56, 57]. Previous studies have indicated the use of a texture analyzer using a 500N load cell, needle penetration of 3.6 mm, velocity of 1 mm s^-1, and a trigger force of 0.05N [58]. Although this method is able to provide some quantitative data, it lacks in its relation to product performance when in use by consumers. Human panels are typically used in order to subjectively assess the residual left on the lips and product feel after application. Because this value is subjective, it is subject to change and result in reproducibility issues, making it even more difficult to correlate lipstick hardness with customer satisfaction and performance. In efforts to alleviate this issue, a method and apparatus has been developed to better correlate hardness and deposition to user experience. This method studies different deposition rates of semisolid lipsticks when applied to a substrate under controlled conditions and relates the drag and deposition to consumer experience [59]. It is claimed that the results from this method relate to the consumers ‘mouth feel’ through the relation of consumer preference in terms of lipstick coverage with the deposition accomplished by the invented apparatus [59]. As a result, this creates an in vitro method which better correlates with results from human panels.

Hardness testing is only conducted for formulations that are in the stick format. In long-wear lipstick formulations, the isododecane acts as a good solvent for polyethylene wax; this wax is used to help soften the stick and is difficult to crystallize. These long-wear lipstick sticks, such as Revlon ColorStay and Maybelline SuperStay Ink Crayon Lipstick, are made slim to keep the stick softer without compromising ease of application and preventing breakage of the stick. Should a stick be wide and kept soft, it would not be able to support the stress of application and would be easy to break. Additionally, sticks with a larger diameter enable more evaporation of the volatile solvent due to its larger surface area, making the formulation tacky. In the hardness testing described above, a slim stick should have a readout of 70-100 gForce hardness, making it ideal to deposit a coating onto the lips that is not too thick.