Google Answers Logo
View Question
Q: Flat Iron - Scientific Principles ( Answered,   0 Comments )
Subject: Flat Iron - Scientific Principles
Category: Science > Chemistry
Asked by: zgall101-ga
List Price: $10.00
Posted: 14 Dec 2004 17:31 PST
Expires: 13 Jan 2005 17:31 PST
Question ID: 442711
Can you find a website that explains the scientific principles behind
how a flat iron/straightening iron works? It is for a grade 7 science
project so please try and keep it at that level. If there is nothing
at that level, then give me whatever you can find.
Subject: Re: Flat Iron - Scientific Principles
Answered By: redhoss-ga on 14 Dec 2004 20:40 PST
Hello zgall101, my curiosity got the best of me on this question. It
was not an easy answer to find, but I think I found just what you

Hot pressing hair 

Hot pressing or combing to straighten hair was developed around 1910
and since then has become a very popular practice. Initially conducted
exclusively in salons and beauty parlors, the technique is now popular
for at home use. The method is straight forward if time consuming.
First the hair is washed and partially dried with a towel. A small
amount of pressing oil is combed through the hair. This oil is for
lubrication to allow the comb to pass through the hair more easily and
also to act as a conductor of heat from the comb to the hair. A metal
pressing comb is heated to between 300 and 500 degrees Fahrenheit and
is passed quickly through the hair. The high temperature breaks the
biochemical disulfide bonds between and within the keratin proteins
and allows the hair to be straightened through the tension applied to
the hair during the combing procedure. After the comb has passed
through the hair the temperature drops rapidly and this allows the
broken biochemical bonds in the hair to reconnect and fix their new
position. This reformation of the bonds holds the hair in its new,
straightened shape.

This hair straightening method will hold the hair in a new shape for a
while, but it is not a permanent procedure and the hot combing has to
be reconducted at regular intervals to maintain the look. Exposure of
the hair to water, high humidity and even to excessive perspiration
can cause the hot combed hair to revert back to its original curly
shape. There is then a temptation to overuse the hot comb process, but
this can lead to dry and damaged hair as the cuticle will break down
and expose the softer underlying cortex if it receives too much heat
too frequently. If the comb is too hot or applied for too long the
hair can be burned. This causes severe damage and leads to patches of
hair where breakage occurs and can give the appearance of patchy hair
loss. The most significant risk from hot combing is skin burns. With
care in application skin burns can be avoided, but if the heat is
applied to the scalp for too long skin burning will happen. If the
burn is severe enough it may damage the hair follicles in the skin. In
response the hair follicles may shut down and enter telogen until the
skin and affected hair follicles are repaired. If the burn is really
bad it may permanently damage the hair follicles.

This may be more than you really want to know, but I thought it was
interesting. I also found this concerning hair structure.

In the helical protein of hair, hydrogen bonds within individual
helices of keratin, and disulfide bridges between adjacent helices,
impart strength and elasticity to individual hairs. Water can disrupt
the hydrogen bonds, making the hair limp. When the hair dries, new
hydrogen bonding allows it to take on the shape of a curler. Permanent
wave solutions induce new disulfide bridges between the helices.
Genetically determined, natural curly hair also has a different
arrangement of disulfide bridges compared with straight hair.

This adds support for the other information and gives some new info too.

Here is a good article on the structure of hair.

What is hair fiber made of 

The primary component of hair fiber is keratin. Keratins are proteins,
long chains (polymers) of amino acids. Keratin proteins form the
cytoskeleton (miniature skeleton within a cell) of all epidermal
cells. Keratin filaments run within a cell from the inside of the
outer membrane to weave a "basket" around the nucleus of the cell.
Keratins are a principle part of the cells in the epidermis, hair,
nails, feathers, hooves, horny tissues and the enamel of teeth.

Keratin accounts for 30% of the cellular protein of living epidermal
cells and 85% of cellular protein of the dead cells in the stratum
corunem (the scaly, dead or horny layer of the outer skin). In hair
fiber, keratin proteins comprise 65-95% of the total hair fiber by
weight. Each molecule of keratin is very small - about 10 nanometers
across. To give you a sense of how small that is, 10,000 keratin
molecules lined up side by side would cover the width of the average
terminal scalp hair.

There are several subtypes of keratin proteins, some are called "soft"
keratins and others are "hard" keratins. Basically, soft keratins are
found in the skin and are relatively easy to break down whereas hard
keratins are very resistant to degradation. Hard keratins do not
dissolve in water and they are highly resistant to proteolytic
enzymes. As you might have guessed, hair fiber is mostly made from
hard keratin proteins.

Hair contains a high amount of sulfur because the amino acid cysteine
is a key component of the keratin proteins in hair fiber. The sulfur
in cysteine molecules in adjacent keratin proteins link together in
disulfide chemical bonds. These disulfide bonds are very strong and
very difficult to break apart. These disulfide chemical bonds linking
the keratins together are the key factor in the durability and
resistance of hair fiber to degradation under environmental stress.
They are largely resistant to the action of acids but the disulfide
bonds can be broken apart by alkali solutions. This property is
exploited in perms and chemical hair straightening processes. Alkali
is used to break the disulfide bonds in the keratins and make the hair
weak. The hair is formed into a new shape and then acid is applied to
neutralize the alkali and enable the disulfide bonds to reform. The
reformed disulfide bonds hold the hair in its new shape.

Some cosmetics manufacturers claim to make hair products that contain
keratin to strengthen hair or promote hair growth. However, the cells
in hair follicles make all the keratin protein that they require from
amino acids supplied via the blood. Eating keratin protein does not
help hair growth. Eating hard keratins as found in hair fiber is
pointless as they cannot be broken down and absorbed. The keratins
simply pass straight through the gut. It would be much better to eat
the constituent amino acids from which the hair follicle cells can
make keratin. However, it is extremely rare in North America and
Western Europe to have a deficiency in amino acids so severe that it
affects hair fiber quality. Topically applying keratin to hair also
has no long lasting effect. The keratins are mostly washed away in the
shampooing process.


What amino acids are present in hair fiber 

Below is a table of the amino acids present in normal human hair in
order of their quantity.

Amino Acid Amount in residues extracted 
Cysteine 17.5 
Serine 11.7 
Glutamic acid 11.1 
Threonine 6.9 
Glycine 6.5 
Leucine 6.1 
Valine 5.9 
Arginine 5.6 
Aspartic acid 5.0 
Alanine 4.8 
Proline 3.6 
Isoleucine 2.7 
Tyrosine 1.9 
Phenylalanine 1.4 
Histidine 0.8 
Methionine 0.5 


What is the carbon, oxygen, hydrogen, nitrogen, and sulfur content of hair

In terms of raw elements, on average, hair is composed of 50.65%
carbon, 20.85% oxygen, 17.14% nitrogen, 6.36% hydrogen, and 5.0%
sulfur. Hair also contains trace amounts of magnesium, arsenic, iron,
chromium and other metals and minerals.


How many atoms are present in the average hair fiber diameter 

The average scalp hair fiber diameter is between 80-120 micrometers.
Hair fiber diameter varies depending on the location of the hair
follicle, the ethnicity of the individual and the activity of hair
diseases. An atom has a diameter 10 to the minus 10 meters. That's
0.0000000001m or 1 preceded by 9 zeros and a decimal point. So the
diameter of the average hair fiber is ... er , umm, er ... (extensive
counting of fingers and toes) ... 800,000 - 1,200,000 atoms.

So the cross sectional area of a hair fiber asuming the cross section
is circular (which is not always the case as curly hair as an oval
cross section while straight hair has a circular cross section)
contains 500,000,000,000 - 1,000,000,000,000 atoms. Remember the area
of a circle is pi times the radius squared.

Hope this gets you an A+, Redhoss
There are no comments at this time.

Important Disclaimer: Answers and comments provided on Google Answers are general information, and are not intended to substitute for informed professional medical, psychiatric, psychological, tax, legal, investment, accounting, or other professional advice. Google does not endorse, and expressly disclaims liability for any product, manufacturer, distributor, service or service provider mentioned or any opinion expressed in answers or comments. Please read carefully the Google Answers Terms of Service.

If you feel that you have found inappropriate content, please let us know by emailing us at with the question ID listed above. Thank you.
Search Google Answers for
Google Answers  

Google Home - Answers FAQ - Terms of Service - Privacy Policy