Do Cold Water Rinses Make Hair Shine?

Ever hear this one:
"Rinse your hair with ice cold water to close the cuticle to make it shine."

Ever try it? 

And then there's the opposite - that very warm water makes your hair's cuticles "open up."

I don't get along with the notion that "cuticles open and close" like doors or window blinds is a little inaccurate as a mental image. If a cuticle is elevated, it might be broken and therefore stuck in that position like a damaged roof-shingle. When hair swells in water, cuticles may be lifted as the increase girth of the hair makes it difficult for them to lie flat.

I had to test it myself and I'll admit my tests are not complete (if I spent all day working with 50 to 100 hair samples and perfectly controlled conditions that would be ideal. I'll leave that to people who have money to do this sort of thing.

The idea that "Cold Water Makes Hair Shine" is a great idea in theory. But when you consider different hair colors, different hair textures, different degrees of scalp-oiliness, different curl patterns, different hair densities... It's way too many variables for a pat answer.

Even if we tested the daylights out of some hair swatches, all those variables still have not been accounted for unless we reproduce them all in the lab.

Shine breakdown: I'm going to be looking for a change in porosity because "closing the cuticles" implies reduction of porosity. Shiny hair is lower porosity hair or hair which has had it's porosities temporarily filled in with conditioners or emollients and proteins. 

There is another element of shine which has less to do with cuticles and porosity. When the hair as a whole has a greater reflective surface, hair shines. In order to do that, hairs need to line up neatly with each other. This has more to do with curl pattern, the "slip" in hair products you used, whether you wear your hair is worn straight or smoothly coiffed or tousled or worn naturally curly, and whether your individual hairs are smooth or textured.

There are some standard facts about hair and oils and heat that apply to this question:

1) Warm to hot water assists in removal of oil and soil from hair - including sebum - with but also without detergent.

2) It is more difficult to remove oil and soils (dust, dirt) in cold water. That's why cold-water laundry detergents showed up on store shelves in the 2010s, and now most laundry detergents are cold-water-appropriate.

3) Heat: Dry heat causes cracks in cuticles which tend to develop into chips and ultimately break away as they wear and weather. Hot water heats in a different way which is not necessarily well-studied in hair because hair product manufacturers are more interested in shampoos and conditioners and styling products and curling or straightening irons and blow dryers than hot water.

4) Warm water (heat) speeds up reactions such as diffusion of solutes (proteins, amino acids) in and out of hair; heat liquefies and "thins" more solid or more viscous oils. Things happen faster in heat, in this case oils and proteins are lost from your hair or removed if that's what you're after.

For this project I chose low, normal, and higher porosity hairs and photographed them in room temperature water, in very cold water and in very warm (shower temperature) water.

General impressions:

Cold Water (roughly 45-50°F / 10°C): There is little difference in the hair between room temperature to cold water. Some hairs seemed to have a more dramatic swelling reaction in cold water than room temperature water.  No hairs seemed to show a reduction in porosity which would be indicated by flatter-lying cuticles when viewed on the "edge" or by decreased swelling in water compared to room temperature water.

Warm-hot water (roughly 100°F / 38°C):  In warmer water, some hairs are evolving bubbles and showing increased swelling. Proteins and oils can diffuse in and out via porosities in the hair more readily so the very warm (hot) water speeds up any swelling with water that will occur. We see bubbles evolving around hair when water is moving into the hair and things inside the hair are moving out. So maybe the hair is more porous in the warm water, and certainly porous hair is especially vulnerable in warm to hot water. Note: I had to use such hot water to keep a high-enough temperature under the microscope.

Conclusions:

Cold water 🧊  has the ability to produce mild hair-swelling. This may be more pronounced in porous hair. But the swelling is less than in hot water.

Cold water may be less dehydrating to hair as a result. Especially to dry or damaged hair.

Hot water 🔥 may cause hair to swell more rapidly than room temperature water. Swelling in hair has an ultimate side-effect of elevating the cuticles so they stand out more than usual. 

Hot water may be more dehydrating to hair, as swelling can allow water to wash amino acids and lipids out from under the cuticle. Down the drain they go. This is especially true for already-dry, dyed, highlighted, or heat-styled hair. 

Hair returns to its original state when its temperature returns to normal or pre-test appearance.

My impression: Keep out of the really hot showers and save "ice-cold water over the head" for the coaches of winning football games. Or would that be Gatorade? Or would that be basketball?

If you swear cold water works for you - go for it. I don't have any love for being in ice-cold water.  I could not find any research supporting the cold water rinse it (and I probably never will - it's not exactly a marketable product). 

Above is a kinking, curly hair (curl is pencil-sized, kinking is fairly close together) of low-normal porosity in the room temperature, cold, and hot water treatments (left, center, right, respectively). In the "before" room temperature photo at left, the cuticle lies closely to the hair. In the cold water photo (middle) the blue arrows indicate elevated cuticle scales where the hair is probably swelling during the shock of the cold water. At right the blue arrows indicate where the hair is swelling in the hot water.

Coarse hair with low porosity in room temperature water (left). Cuticles lie flat and tight to the hair surface. In cold and hot water, there is little change. This seems to be common in coarse, low porosity hair, it tends to resist just about everything. And that is a good thing. Especially if you like hot showers.

Porous hair in room temperature water (left) and cold water (middle). There is little difference in the elevation of cuticles between these two temperatures. At right in hot water, the hair is swelling more, the cuticles are slightly more elevated and tiny bubbles are evolving at the hair surface.