The Effect of Magnesium Carbonate on the Human Finger Pad

THE EFFECT OF MAGNESIUM CARBONATE ON THE HUMAN

FINGER PAD

1

Andrew Dominguez

Mechanical Engineering Department, Loyola Marymount University, Los Angeles, CA 90045, USA

Abstract: The main purpose of this paper is to illuminate the current state of understanding of

the effect of magnesium carbonate, otherwise known as climbing chalk by rock climbers, on the

human finger pad. Climbing chalk is used by rock climbers to dry their hands in order to increase

their ability to grip climbing holds. There have been two conflicting papers written specifically

on answering if chalk increases or decreases the coefficient of friction of the finger pad: Use of

‘chalk’ in rock climbing: sine qua non or myth? and The effect of chalk on the finger–hold

friction coefficient in rock climbing. This paper will review the method and results of each study

and look into several factors that need to be studied in more depth to conclusively understand the

interactions that are happening at the finger-surface interaction.

Key Words:

Chalk, Climbing, Coefficient of Friction, Friction, Finger, Grip, Magnesium Carbonate, Rock

Climbing, Solid Lubrication

INTRODUCTION

Rock climbing is a sport in which

participants climb up, down or traverse across

natural rock formations or artificial rock walls.

The goal of the sport is to reach the summit or

endpoint of a formation. Climbing has recently

become a very popular sport pursued by many

in a professional and recreational activity. It is

a physically and mentally demanding sport that

tests a climber's strength, endurance, agility

and balance along with mental control. The

ability to maintain contact with holds is a focal

point for climbers as they exert high-intensity

forces on their hands and feet. This skill is

paramount to the performance of climbers

especially when there are holds with a limited

area for finger or foot placement. Climbers

often use magnesium carbonate, or climbing

chalk, to help keep grip to the rock in high

strain climbs.

Magnesium carbonate is traditionally

carried in a bag that is attached to a climber’s

waist. Climbers dip their hands in to cover them

with chalk with the intent that it will dry up

their sweat and improve their grip on the holds.

The application of chalk has become

ubiquitous with rock climbing and almost

universally seen as a means to increase grip by

reducing moisture present on the hands. The

coefficient of friction can be altered by the

introduction of substance between the surfaces

in much the same way that liquid or solid

lubricants are used. Some influences that the

rock climbing communities have commonly


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recognized that effect the finger-hold

interactions are: transpiration, rock type,

ambient temperature and humidity [1].

RESULTS

Two main studies have been completed

discussing the effect of chalk on the friction

coefficient between the climber’s fingers and

the climbing surfaces, but neither has definitely

proven its effects. No paper has been written to

scientifically examine the effects that are at

work during the hand, chalk, and rock surface

interaction, though several papers have been

conducted on similar and related topics

especially in the field of the effects of sweat.

This paper was written discuss current

understanding on the tribological interactions

in climbing chalk and to bring to attention

possible explanations of what interactions are

happening. The two journals written on the

subject in question are: “Use of ‘chalk’ in rock

climbing: sine qua non or myth?” and “The

effect of chalk on the finger-hold friction

coefficient in rock climbing”.

The former paper, “Use of ‘chalk’ in

rock climbing: sine qua non or myth?” came to

the conclusion chalk had an adverse effect on

the coefficient of friction on fingers, contrary

to popular belief. The author, Li, attempted to

disprove an assumption by many climbers that

“dry skin grips better, chalk dries the skin, so

by regular application of chalk one increases

the coefficient of friction between the skin on

the hands and the climbing surfaces.” Li

conducted the ‘beginning slip method’ for

experimentation which consisted of fifteen

people that were asked to use their fingers to

apply a sufficient normal force on a flattened

rock to prevent slippage. [12] They were then

asked to gradually reduce pressure until

slippage occurred and at this instant the normal

and tangential forces were measured to find the

coefficient of friction. The experiment

compares chalk versus no chalk, damped

versus not damped, and used sandstone,

granite, and slate to simulate the climbing rock

environment. From that experimental set-up,

the coefficient of friction was calculated by

using the ratio between the tangential force and

the normal force. The experimental set-up is

displayed in Figure 1 and the main results are

displayed in Figure 2 and Figure 3. The results

from this experiment, contradicted perceived

climber’s beliefs that the coefficient of friction

between their fingers and the climbing surfaces

would increase. [12] The reasons postulated by

the author for chalks decreasing results are

explained by two independent causes. The first

cause is that magnesium carbonate absorbs the

moisture in the skin causing the fingers to dry

reducing the fingers real area of contact.

Secondly, the chalk when applied on dry hands

behaved as a slippery granular layer similar to

that of a dry lubricant, thus decreasing the

coefficient of friction. [12] This paper only

postulated these reasons and did not try to

scientifically prove either, but only aimed to

show that magnesium carbonate decreased the

coefficient of friction. Furthermore, when

comparing the simulated rock surfaces, it was

determined that sandstone was less slippery

than granite and slate. Figure 4 shows the

graph that clearly shows how sandstone is the

roughest of the three surfaces and is probably

due to the hydrophilic nature of sandstone

compared to the hydrophobic nature of granite

and slate. In conclusion, this experiment

observed that chalk decreased the coefficient of


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friction which would decrease the climbers grip

while climbing, dampness of hands had no

effect on the coefficient and that sandstone had

the highest coefficient of friction.[12]

Contrary to the previous paper, another

was published, “The effect of chalk on the

finger-hold friction coefficient in rock

climbing”, claiming that the coefficient of

friction did indeed increase between the contact

surface and the climbers fingers when

magnesium carbonate was applied on the

fingers. The author, Amca, thought that the

previous research done on chalk did not

account for the stresses that occur during rock

climbing. [1] His main critique of the previous

experiment is that a finger would experience a

much stronger resistant force than 3.5kg in a

real climbing situation. Amca believed that

Li’s method would not take the importance of

the distortion of the skin of the human finger

and the resulting change of form and

characteristics in viscoelastic material

properties. This study was conducted in vivo

and tried to mimic real climbing environments.

Two common climbing surfaces chosen to

simulate the rocks from which climbers hold

were, sandstone and limestone. In order to

complete this research, eleven climbers

completed a total of 42 test sessions in which

every climbers hung from four fingers in a

natural position on a specially designed hang

board. The experimental test set-up can be seen

in Figure 5. The inclination of the holds then

increased until the climbers slipped. In order to

calculate the coefficient of friction at the

moment of slip, a gyroscopic sensor was used.

The results from this experiment showed a

favorable effect of chalk on the coefficient of

friction. [1] Where, the coefficient of friction

increased by 18.7 percent for a limestone

surface and it increased by 21.6 percent for a

sandstone surface.[1] Figure 6 illustrates the

results for the mean coefficient of friction

values and standard deviation values for both

chalk and rock conditions In conclusion, this

particular experiment proved in favor of the

climber’s theory, which is believed that chalk

enhances friction, allowing for a better grip.

This study also was only to investigate the

hold-finger contact and did not aim to find the

actual factors which led to these results. The

author hypothesizes that the better

performance observed with chalk can be

explained by several possible reasons:

“modifications of the skin roughness,

modification of skin elasticity which enables

the fingers to best adapt to the hold shape and

changes in water/sudation elimination

behavior.” [1]

DISCUSSION

The aim of this discussion is to clarify

the important factors that might determine the

friction of the finger pad and provide an

argument that the results of the two papers are

not in opposition, but in accordance with one

another and both provide part of a fuller

explanation to what is actually happening

between finger and rock surfaces. A very wide

range of coefficients of friction have been

reported for human skin, especially in the

presence of friction modifiers such as

lubricants. The presence of water is most likely

the largest effect on the coefficient of friction

of the human finger pad as seen by authors

Andre´ et al. [2] It can be in due to internal or

external influences and both need to be

considered in tribological contacts such as


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condensation on rock formations or the

excretion of sweat through the finger pad.

Sweat is a hypotonic solution that is 99% made

up of water [9]. The presence of sweat can

greatly modify the coefficient friction between

the hands and contacting surfaces, thus it is

very important to understand how moisture on

the hands can affect the finger pad surface

interactions. Measuring the effect of sweat on

skin-surface interface is not so simple due to

the large variety of factors in play, especially

discrepancies between different persons’ sweat

rates. [3] Body site is also very imperative to

this as the stratum corneum, or outer most layer

of skin on the finger, acts very different than

other layers of skin such as the forearm

investigated by Kwiatkowska et al. [8] Also,

persons’ sweat rates are due to mental and

physical states of stress. [3] While many affects

are difficult pinpoint with certainty because

there is much variation between people and the

how their bodies act during different activities,

there are noticeable trends present. [11]

A survey of fingertip moisture was

conducted by S. E. Tomlinson et al. at the

University of Leeds, Sheffield and York, to

investigate the effect of moisture on the friction

of the human finger pad. [11] It was found that

the as the level of moisture increased on the

finger, the friction also increased up to a certain

level of moisture and then decreased. This can

be seen in Figure 7 below. It has also has been

shown elsewhere that highly wet or highly dry

skin showed relatively low friction [2] . Heavy

exercising results in an amount of moisture past

a critical threshold of moisture. The coefficient

of friction reduces as the stratum corneum

enters the mixed lubrication regime, between

the boundary lubrication and IEHL region and

reduces friction and decreases the ability to

grip. [11] Friction in a relatively damp interface

can be considerably reduced by decreasing or

increasing the interfacial moisture content. [3]

Therefore an optimal use of the chalk is

important in order to keep the hand in the ideal

moisture range. [1]

As stated by Tomlinson, the “increase

in friction has been previously been explained

by viscous shearing, water absorption and

capillary adhesion”. [11] Some other

explanations as to the increase in friction due to

moisture have been attributed to occlusion,

plasticization and normal force at small loads.

Their study partially verified this hypothesis

and found that water absorption and capillary

adhesion are the two principle mechanisms

responsible for the increase in friction while the

effect of viscous shearing of liquid bridges is

negligible or not present, but worth further

exploration due to an inability to directly test

for it. [11] The reason why water absorption

increases the finger pad’s coefficient of friction

is that the pad becomes suppler, increasing the

real contact area. [11] All materials, especially

the finger pad, is made up of a series peaks and

valleys of material and as water is absorbed

into skin these ridges become more even

meaning more of the skin will come into

contact. [4] Capillary adhesion is a very similar

phenomenon that increases the real area of

contact of the finger pad with increased levels

of moisture. It works by pulling the walls of the

ridges of the finger into closer contact. This

interaction is highly dependent upon the

Young’s modulus of the stratum corneum

which is regulated the amount of moisture

present on the skin. Absorption of water in the

stratum corneum will reduce the Young’s


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modulus and in turn increase the effect of

capillary adhesion. Liquid bridges, a concept

introduced by Dinç et al. are the formation of a

meniscus that bridges between the ridges on the

finger in contact with each other as seen in

Figure 8. It was proposed that liquid bridging

occur as moisture levels increase causing

increased friction from viscous shearing

effects. [6] Tomlinson in his investigation

concluded though that the effect of viscous

shearing in the liquid bridges has negligible

effect, but that there were many limitations in

the modeling could use further exploration.[11]

Moisture accumulation as a result of

skin surface occlusion by the counter body is a

very important factor on the coefficient of

friction. Occlusion inhibits the evaporation of

sweat from the skin surface leading to an

increase in the moisture level of the stratum

corneum and at the interface.[3] The effect is

exacerbated because of the density of sweat

pores located along the papillary ridges of the

finger pad. Occlusion is a time based factor; the

longer the finger pad is present against a

surface the more sweat will accumulate and

reach the critical moisture threshold. In

particular, it is found that the coefficient of

friction is smallest at short occlusion times,

corresponding to a relatively dry skin surface

and can be prominently seen in Figure 9. [3]

The coefficient increases with increasing

contact time to a steady-state value that is

greater than that in the wet state. [3] This

reaffirms many authors’ observations of a bell

shaped friction to moisture curve where the

maximum friction is in a damp state. Skin

occlusion might also affect the interface

temperature which could affect the interfacial

shear strength and viscoelastic mechanical

properties of the stratum corneum and hence

the friction, but this has yet to be tested. [3] As

skin temperature rises, its pliability increases,

since the lipid bilayer of the cell wall becomes

more lucid thus increasing its real area of

contact and coefficient of friction. [12] Amca,

on the other hand, did not find any significant

correlation between humidity, temperature and

friction coefficient. [1]

CONCLUSION

The skin rock interaction is much more

complicated than it at first appears and is

dependent upon many factors that are not

properly addressed in any research done to

date. It appears that climbing chalk can indeed

both increase and decrease the coefficient of

friction of the human finger if the right

conditions are met. Matt Carre in the journal

“An Assessment of the performance of grip

enhancing agents used in sports applications”

assessed the performance of four grip

enhancement agents: powdered chalk, liquid

chalk, rosin, and turpentine. He found that in a

dry environment powdered chalk actually

decreased the coefficient of friction. [5]

Another finding was that only the granular

powder-based agents increased the coefficient

of friction on wet fingers [5]. This agrees with

both Use of ‘chalk’ in rock climbing: sine qua

non or myth? and The effect of chalk on the

finger–hold friction coefficient in rock

climbing. The reasons for the discrepancies

between the first two papers most likely have

to do with the amount of water present in the

stratum corneum and between the finger pad

and rock surface. Because of the bell shaped

trend between moisture and the coefficient of


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friction an optimal use of the chalk is important

in order to keep the hand in the ideal moisture

range. This means that the use of chalk is

situational and over chalking ought to be

avoided as not to dry out skin too much, but is

necessary to achieve optimum friction in some

high strain environments where too much

sweat is being excreted. Research is still

necessary to clarify to what extent water

absorption, capillary adhesion, occlusion,

normal force, skin roughness and plasticization

play in the coefficient of friction. Though, it is

observed that the coefficient of friction is less

dependent upon the liquid bridges, differences

in larger normal forces, temperature and

humidity.

REFERENCES

[1] Arif Mithat Amca , Laurent Vigouroux ,

Serdar Aritan & Eric Berton (2012): The effect

of chalk on the finger–hold friction coefficient

in rock climbing, Sports Biomechanics, 11:4,

473-479

[2] André, T., Lefevre, P., Thonnard, J,-L.: A

continuous measure of fingertip friction during

precision grip. Journal of Neuroscience

Methods 179, 224-229 (2009)

[3] Asumarty, Subrahmanyam M., Simon A.

Johnson, Simon A. Watson, and Michael J.

Adams. "Friction of the Human Finger Pad:

Influence of Moisture, Occlusion and

Velocity." Tribology Letters 44.2

(2011): 117-37. Print

[4] Bhushan, Bharat. "Principles and

Applications of Tribology." Ebrary. William

H. Hannon Library, n.d. Web. 26 June 2013.

[5] Carre, Matt J. "An Assessment of the

Performance of Grip Enhancing Agents Used

in Sports Applications." Journal of

Engineering Tribology (2012): n. pag. Print.

[6] Dinç, O.S., Ettles, C.M., Calabrese, S.J.,

Scarton, H.A.: Some parameters affecting

tactile friction. Journal of Tribology 113, 512-

517 (1991)

[7] Ford, Matt. "Characterization of Climbing

Chalk." (n.d.): 1-8. Web. 16 June 2013.

<http://mattfordengineering.files.wordpress.co

m/2012/05/lab06.pdf>.

[8] Kwiatkowska, M., Franklin, S.E., Hendriks,

C.P., Kwiatkowski, K.: Friction and

deformation behavior of human skin. Wear

267, 1264–1273 (2009)

[9] Marieb, E. (1992). Human Anatomy and

Physiology, 2nd edn. Redwood, CA: Benjamin

Cummings.

[10] Seo, N.J., Armstrong, T.J., Drinkaus, P.:

A comparison of two methods of measuring

static coefficient of friction at low

normal forces: a pilot study. Ergonomics 52,

121–135 (2009)

[11] Tomlinson, S.E., Lewis, R., Liu, X.,

Texier, C. and Carre, M.J. (2011)

Understanding the friction mechanisms

between the human finger and flat contacting

surfaces in moist conditions. Tribology Letters,

41 (1). pp. 283-294. ISSN 1023-8883

[12] Use of ‘chalk’ in rock climbing: sine qua

non or myth?F.-X. Li, S. Margetts, I. Fowler

Journal of Sports Sciences Vol. 19, Iss. 6, 2001


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FIGURES

`

Figure 1: Experimental test set-up for “Use of ‘chalk’ in rock climbing: sine qua non or myth”

[12]

Figure 2: Effect of chalk on the coefficient of friction. [12]


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Figure 3: Effect of chalk and water on the coefficient of friction. [12]

Figure 4: Effect of sandstone, granite and slate on the coefficient of friction. [12]


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Figure 5: Experimental test set-up for “The effect of chalk on the finger-hold friction coefficient

in rock climbing”. [1]

Figure 6: Effect of chalk on the coefficient of friction. [1]


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Figure 7: Effects of moisture on the coefficient of friction on the human finger pad. [3]

Figure 8: Liquid bridges formed in the ridges of the human finger. [6]


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Figure 9: The required later force needed over 140 seconds. This increase correlates to an

increase in the coefficient of friction as occlusion sets in. [3]

Documents

The Effect of Magnesium Carbonate on the Human Finger Pad