You have developed a cream formula that looks perfect on the day you make it. But what will it look like in a month? In three? What about when stored in a bathroom where the temperature fluctuates from +15°C in winter to +30°C in summer? This is exactly why stability testing exists — and a significant part of it can be done at home, without expensive equipment.
Why test for stability at all?
An unstable formula is not just an aesthetic issue. It means:
Separation and breakdown of the emulsion — the product becomes unsuitable for use
pH changes — active ingredients lose their effectiveness or become irritating
Oil oxidation — rancid odour, formation of free radicals
Microbial growth — if the preservative cannot cope with the changed environment
Changes in colour and scent — a signal of ingredient degradation
Professional laboratories conduct tests for years in climate chambers with precise environmental control. At home, we cannot fully replicate these conditions, but we can gather enough information to make an informed decision about our formulation.
If you haven't mastered pH yet, we recommend starting with our guide to pH in cosmetics. pH control is one of the key stability tests.
Test #1: Real-time visual assessment
Difficulty: ⭐☆☆☆☆ | Equipment: eyes, notebook
The simplest yet highly informative test. The essence is to observe a sample under several storage conditions in parallel.
How to conduct it
Immediately after mixing, divide the product into at least 3 samples in identical transparent containers:
Sample | Storage conditions | What it simulates |
|---|---|---|
A | Room temperature (~20–22°C) | Standard storage |
B | Fridge (+4–6°C) | Cold storage, winter |
C | Warm place (+30–40°C) | Summer, bathroom |
Evaluate the samples on days 0, 3, 7, 14, 30, 60, and 90, and record: appearance, colour, scent, texture upon application, and the presence of separation, water droplets, graininess, or bubbles.
Photograph the samples on the same day, against the same background, and in the same lighting. A month later, you will be grateful to yourself for this — changes in colour or texture that are not noticeable to the naked eye become clearly visible when comparing photos.
Test No. 2: Freeze-Thaw Cycle
Difficulty: ⭐⭐☆☆☆ | Equipment: freezer, room temperature
One of the standard accelerated stability tests that is easy to replicate at home. It checks whether an emulsion can withstand sharp temperature fluctuations — for example, during winter transport or storage in a cold warehouse.
How to conduct it
One cycle = 18 hours in the freezer (−10 to −20°C) + 6 hours at room temperature. Perform at least 3 cycles (ideally 5–6). Evaluate the sample visually after each cycle.
Interpreting the results
✅ Passed 3 cycles without changes — good basic stability
✅ Passed 6 cycles — the cream formula is resistant to transport stress
❌ Separated after 1–2 cycles — the emulsifying system needs to be revised
Some W/O formulas with a high wax content may change texture when frozen but recover after thawing. This does not always mean instability — evaluate the final appearance after full recovery at room temperature.
Read more about stabilising water-in-oil emulsions in the article Table salt vs. magnesium sulphate: how to stabilise a water-in-oil emulsion.
Test No. 3: Accelerated ageing (thermal stress)
Difficulty: ⭐⭐⭐☆☆ | Equipment: oven or dehydrator with a precise thermostat
At home, you can use an oven on its lowest heat setting or a dehydrator — the main thing is that the temperature remains stable.
Principle
Every 10°C increase in temperature approximately doubles the rate of chemical reactions (the van 't Hoff rule):
+40°C
1 month at +40°C ≈ 2 months at +20°C
+50°C
1 month at +50°C ≈ 4–6 months at +20°C
How to conduct it
Place the sample in a tightly sealed container. Keep it at +40°C for 4–8 weeks, checking weekly for: appearance, odour (rancidity is a sign of oil oxidation), colour, and pH.
Use glass containers for the +40°C test — during prolonged heating, plastic can leach substances into the formula.
Test No. 4: pH control
Difficulty: ⭐⭐☆☆☆ | Equipment: pH meter (recommended) or pH strips
pH is one of the key indicators of stability. A change in pH indicates that chemical reactions are occurring in the formula: oxidation, hydrolysis, or the degradation of actives or preservatives.
When to measure
Day 0 (immediately after mixing) — baseline value
Day 7, 14, 30 — during normal storage
After each Freeze-Thaw cycle
After each week of thermal stress
Permissible deviations by product type
Product type | Target pH | Permissible deviation |
|---|---|---|
Moisturising cream | 5,0–6,0 | ±0,3–0,5 |
Vitamin C serum | 2,5–3,5 | ±0,2 |
Shampoo | 4,5–5,5 | ±0,3 |
Toner / lotion | 4,5–6,0 | ±0,3 |
Cleansing balm | 5,0–6,5 | ±0,5 |
pH strips provide a rough estimate with an accuracy of ±0.5, which is insufficient for working with narrow-range actives (ascorbic acid, AHAs, peptides). For serious work, invest in a basic pH meter — it costs from around £15 and pays for itself with the very first corrected formulation.
Read more about buffer systems for pH stabilisation in the article Glucono-delta-lactone → Sodium gluconate: how to make a buffer for an emulsion.
Test No. 5: Centrifugation
Difficulty: ⭐⭐⭐☆☆ | Equipment: laboratory centrifuge or alternative
Centrifugation is a standard express test for assessing the primary stability of an emulsion. It simulates physical phase separation that would occur under real-world conditions over weeks or months.
Protocol
3,000–3,500 rpm for 30 minutes
Evaluate the sample immediately after and again after 24 hours
✅ No separation — good primary stability
⚠️ Slight phase separation, recovers upon shaking — moderate stability
❌ Clear separation — the cream formula requires further development
If you don't have a centrifuge
Use the “vibration test”: place the sample on a running washing machine during the spin cycle for 15–20 minutes. This is a crude analogue, but it allows you to identify the most unstable systems.
Test No. 6: Microbiological stability assessment
Difficulty: ⭐⭐⭐⭐☆ | Equipment: special test strips or an external laboratory
A full Challenge Test requires a laboratory. However, at home, you can conduct a provocation test for a rough assessment of how your preservative is performing.
Simplified protocol
Prepare two identical samples
Introduce “contamination” into one — touch the surface with a clean finger 3–5 times, simulating real-world use
Store both samples at +30°C for 4 weeks
Evaluate weekly: changes in colour, odour, appearance of a film, or cloudiness
This is not a substitute for a professional test. If you are selling a product, microbiological testing in an accredited laboratory is mandatory. The home protocol is for personal use and preliminary assessment only.
For more on choosing and working with preservatives, see the article Preservation in cosmetics: why one preservative won't save your formulation.
Minimum starter kit
Tool | Purpose | Approximate cost |
|---|---|---|
pH meter (basic) | pH control | from around £15 |
Glass jars 10–20 ml | Test samples | £2–4 |
Thermometer (culinary) | Temperature control | from around £3 |
Notebook or spreadsheet | Logging | free |
Smartphone | Visual documentation | already have |
Oven with a low-heat setting | Thermal stress (+40°C) | already have |
Freezer | Freeze-Thaw test | already have |
Total: minimum start — around £20–25. If you already have a pH meter, you have everything else.
How long does testing take?
Duration | What this means |
|---|---|
4 weeks of testing | Minimum for personal use |
8 weeks | For a shelf life of 6–12 months |
12 weeks | A solid foundation for 12–18 months |
Professional laboratory | Mandatory for sales |
Summary
Stability testing is not a tedious chore, but a dialogue with your cream formula. Tests are exactly what help you understand how your formulation behaves in real-world conditions, what needs to be corrected, and when the product is truly ready.
Start small: three samples, a basic pH meter, and a disciplined observation log. This is enough to make informed decisions — and avoid giving users a product that will separate in two weeks.
Read also
Preservation in cosmetics: why one preservative won't save your formulation
Table salt vs. magnesium sulphate: how to stabilise a water-in-oil emulsion
Polymorphism of butters: why cocoa butter is temperamental and how to work with it
Glucono-delta-lactone → Sodium gluconate: how to make a buffer for an emulsion
Oksana Walker
Cosmetic chemist
IFSCC • SCS
