Simulating Glassblowing Steps

Return to Sitemap

2006-11-06 Rev. 2008-03-29, 2009-08-27, 2010-03-27 Page under continuing construction

Simulation Discussion Formulas Viscosity Table
My purpose in building this page is to think about how the handling of glass on the pipe might be simulated on a computer, mostly for curiosity, I guess.  As becomes clear in the notes, this is not easy and I may never get close to a solution 2010-03-27
Glass Simulation Viscosity
My current projects are ambitious dreams and frustrations. When I went to look at the online virtual world, Second Life, I could not even go and try it because it didn't like my otherwise competent video card. When I look at your product, I can't even get a good list of what you do include.
The actual "project" is to simulate the handling of molten glass being taken out of a pool of glass with a pipe and being turned to keep it from dripping and the changing handling with cooling temp and increasing viscosity. The turning would have to be maintained, perhaps with repeated use of a specific key on the keyboard while steering the pipe, perhaps with the up-down-left-right cursor keys. If that were made to work, then a second project might be to inflate a small bubble within the glass and keep it centered while further inflating. More complicated would be to add molten glass to the outside to permit further inflation without getting too thin.
I haven't done any programming on this, although based on my experience and education I could do it with a substantial increase of my knowledge of viscosity physics.

Mike Firth
Furnace Glass Blowing Web Site

MF Notes: Possible modeling
Section drawing of glass flow on rotating pipeCrude model: starting viscosity, increasing value with time, flow and shear formulas
Prime model: viscosity based on temperature, therefore temp of furnace (and pipe?), with decreasing temp a function of radiant and convective loss of heat.
Bubble and Centering
Surface flow of glass on rotating pipeMarvering, blowing, starting bubble or not based on viscosity and internal and surface temp.
Regathering and inflating
Regathering, amount of glass and over or okay heating with collapse
Centering and inflating with wall thickness.

I am just beginning to explore whether I am capable of modeling a simulation of the handling of molten glass as in glassblowing. In looking at your page  I am somewhat overwhelmed by what I have to learn in spite of a background in engineering and a number of years as a computer programmer.
The model for gathering glass is going to be a rotating cylinder (the end of the pipe) turning at a few rpm in a liquid with a beginning viscosity about like that of honey (roughly 3000-6000 cP). In the liquid glass, the temperature of the end of the pipe is a dull red heat at the start and the glass is about 2050F. The pipe rapidly increases in temperature while the glass near the pipe is cooled. The pipe is rotated to gather glass around it and then is removed from the pool of glass. The pipe must be kept rotating to keep the glass from dripping off and to keep it evenly distributed.
Does anything in your models come close to dealing with this kind of situation?
For the record, the next step in my model will be to deal with the change in viscosity as the temperature of the glass rapidly falls, the effect of marvering (rolling the cylinder of glass on a steel surface to chill the glass surface), and blowing a bubble inside the still hot core.

"The temperature lag between the surface and centerline temperature, defined as Tlag=(Ts-Tc)/Tc, describes the thermal response process in the glass.  It is found that the heating zone length affects the lag level only slightly. However, the influence of draw speed is significant. The temperature difference between the surface and centerline temperature increases about 30% when the draw speed goes up from 15 m/s to 25 m/s. This result suggests that the residence time of the glass rod inside the heating furnace is the dominating factor to control the radial temperature difference, since a longer residence time causes the preform to be more uniformly heated.

The radial non-uniformity in the temperature in the perform noticeably affects the velocity field, because glass viscosity is an exponential function of the temperature. This can cause the redistribution of the material dopants and impurities, and therefore impact the fiber quality. Similarly, the axial  velocity lag between the centerline and the surface in the neck-down region is defined as vlag=(vs-vc)/vc. The velocity lag is very sensitive to the furnace dimensions. The lag decreases significantly with an increase in the furnace length."



Absolute viscosity is the characteristic of a fluid which causes it to resist flow. The higher the numerical value of absolute viscosity assigned to a fluid, the greater the resistance that fluid offers to flow.
Viscous Drag
When a viscous fluid flows over a solid surface, a force is exerted on the surface in a tangential direction. In effect, the moving fluid is attempting to drag the solid surface along with it. The magnitude of this force is dependent upon the viscosity and velocity of the fluid.
Experimentally measured drag forces are generally plotted in terms of CD vs. Re on a log-log chart. CD is the drag coefficient and Re is the Reynolds Number. The drag coefficient is defined by:

A plot of CD vs. Re usually looks something like Figure 6. The drag coefficient decreases rapidly with the Reynolds Number in laminar flow, rises abruptly in the transition region, then levels off and eventually decreases slowly in the turbulent region.
Visscosity curve
The following information may have errors; It is not permissible to be used by anyone who has ever met a lawyer.

A sticky subject
We can say that viscosity is the resistance a material has to change in form.  This property can be thought of as an internal friction.

Laminar Flow
To get a good feel for viscosity, we need to first digress and remember laminar flow.  If a fluid or gas is flowing over a surface, the molecules next to the surface (the ones clinging to the walls) have zero speed. As we get farther away from the surface the speed increases.  This difference in speed is a friction in the fluid or gas.  It is the friction of molecules being pushed past each other. You can imagine that the amount of clinging-ness between the molecules will be proportional to the friction. This amount of clinging-ness is called viscosity.  Thus, viscosity determines the amount of friction, which in turn determines the amount of energy absorbed by the flow.

So now how do we measure this resistance?
Imagine a large room filled ankle deep with tar.  On top of the tar is a large plate of steel that we want to slide across the surface. Now lets think of the cube of tar under the plate that is resisting this motion.

Friction is a force (in Newtons) acting against the direction of travel. This frictional force scales with the surface area in (in m2). This friction is also inversely related to the thickness of the tar (in m). We all know that it is easier to stir molasses slowly so we know that the force goes up with  the speed of stirring  (in m/S). With Pl representing the our constant of viscosity we derive it here:

Force = Pl x (Area of plate) x (speed) / (thickness)  
Changing speed to Distance per second distance / time :
Force = Pl x (Area of plate) x (distance) / (thickness) x (time)
Rearranging to solve for the Poise value for tar:
Pl = Force x (thickness) x (time) / [(Area of plate) x (distance)]


Pl = Nms/m3 = Ns/m2

This is viscosity; a unit of power per unit of area, unit of speed and unit of speed gradient. (see this)

The Abbreviation Pl is the SI (mks) unit is called a Poiseuille. Unfortunately, no one uses it; instead they use the old cgs version of this formula expressed in dyne-seconds per cm2 which is called a Poise (it is smaller, thus shorter (;-). Actually, industry uses the centa-Poise or one-hundredth of a Poise because water has a viscosity of 1.002cP which is very close to 1.

So one Poiseuille = Ns/m2 = Pa*s = 10 aPoise = 1000 cP

Jean Louis Poiseuille (1799 - 1869)
The names Poiseuille and the shortened Poise are from the great French physician, Jean Louis Poiseuille (1799 - 1869).
I will digress at this point to try and help you pronounce this unit's name the best I can. Unfortunately, like many of the French, he had a name that is impossible to pronounce or spell. (The problem of pronouncing Poiseuille may well be the main cause of the United States failing to embrace the metric system<grin>)  To start with poise is NOT pronounced as its homograph poise, meaning to walk in a balanced, dignified manner. Instead, imagine you are all puckered up, ready to kiss, just as your partner stomps painfully on your toe. The sound that comes out is "Poise". Phonetically it is spelled pwaz.  (Hint: Be sure to practice in a place where you won't accidentally spit on someone.)  With a little practice you will have mastered the more common of the units.  Don't worry if you don't say it quite correctly; no one else can either or will ever even know the difference.
Now for those up to a bit of a challenge; "Poiseuille".  The first syllable is the same as Poise.  All that is left is uille.  A big help when pronouncing French words is to ignore the sounds of the all the letters.  The syllable uille has no U, I, L, or E sound.  Fill your mouth full with extra viscous peanut butter, plug your nose, and say sores.  Phonetically it is spelled z shwa (r)z 
Probably because no one could pronounce his name, Poiseuille became a hard working student.  Schooled in physics and mathematics Poiseuille developed an improved method for measuring blood pressure.

Poiseuille’s interest in the forces that affected the blood flow in small blood vessels caused him to perform meticulous tests on the resistance of flow of liquids through capillary tubes.  In 1846, he published a paper on his experimental research.  Using compressed air, Poiseuille forced water (instead of blood due to the lack of anti-coagulants) through capillary tubes.  Because he controlled the applied pressure and the diameter of the tubes, Poiseuille’s measurement of the amount of fluid flowing showed there was a relationship.  He discovered that the rate of flow through a tube increases proportionately to the pressure applied and to the fourth power of the tube diameter.  Failing to find the constant of proportionality, that work was left to two other scientists, who later found it to be pi/8.  In honor of his early work the equation for flow of liquids through a tube is called Poiseuille's Law.

(This story is somewhat misleading because blood flow pulsates and the viscosity of blood declines in capillaries as the cells line up single file.  Thus, even with this information we have to wait for the discovery of anticoagulants - (by a long hair, Karl Paul Link, at University of Wisconsin -- vitamin K)  to do definitive experiments.)

Poiseuille's Law of The Flow of Liquids Through a Tube:


l = the length of the tube in cm
r = the radius of the tube in cm
p = the difference in pressure of the two ends of the tube in dynes per cm2
c = the coefficient of Viscosity in poises (dyne-seconds per cm2)
v = volume in cm3 per second


v = pir 4 p/8cl

While the viscosity of solids and liquids falls with increasing temperature (inverse relationship), the viscosity of a gas increases.  While it seems counter intuitive, the rise in gas viscosity as a function temperature can be understood.  As a gas gets higher in temperature it has more collisions, and thus, more friction with its neighboring molecules.

The dependence on pressure of Viscosity
Another surprise is that pressure has very little effect on the viscosity of water.  Water's virtual independence from pressure means that water flowing in a pipe has an insignificant change in friction whether at 60 psi or 20,000 psi!
Water is uniquely self compressed by hydrogen bonding.  At a molecular weight of 18, it is lighter than air and otherwise would be concentrated at the outer limits of the atmosphere.  To understand this compare water to the iso-electronic, and heavier, gas H2S.
Instead, water is a liquid because its tetrahedral arrangement of two protons (positive charges) and two electron pairs (negative charges) makes the molecule polar which causes collections of water molecules to spontaneously arrange themselves into assemblies held together by electrical charges.  This hydrogen bonding force or 'Van der Waals' force is somewhat greater than several thousand psi we might apply.
Oils, and almost everything else, are not so self-compressed.  Paraffinic oils, in particular, are held together by much weaker induced dipole charges.  Thus there is much space between the molecules that allow them to pass easily by each other.  This space can be reduced by applying pressure, increasing density and viscosity. 
This difference is of critical importance to the business of drilling for oil.  When an 8" hole, 4 miles deep is filled with water, the viscosity is essentially the same from top to bottom.  When filled with oil, viscosity and density varies from top to bottom, making it much more difficult to predict friction pressures of flows.

So water and gas's viscosity are mostly independent to pressure.  With Gases until the pressure is less than 3% of normal air pressure the change is negligible on falling bodies.

The Poiseuille and the Poise are units of dynamic viscosity sometimes called absolute viscosity. 
1 Poiseuille (Pl) = 10 poise (P) = 1000 cP
P = 0.1Pl
1 centi-Poise (cP) = .01 poise

Some examples of Viscosity - these may help you get a feel for the cP 
Send in number if you have them
[MF Glass viscosity definition all white entries below from there]
Hydrogen @20°C 0.0086 cP
Ammonia @ 20°C 0.00982 cP
Water vapor @100°C 0.01255
CO2 gas @ 0°C 0.015 cP
Air  @ 18°C 0.0182 cP
Argon @ 20°C 0.02217 cP
Air @ 229°C 0.02638 cP
Neon @ 20°C 0.03111 cP
CO2 Liquid @ -18°C 0.14 cP
Liquid air @ -192.3°C 0.173 cP
Ether @ 20°C 0.233 cP
Water @ 99°C 0.2848 cP
Acetone 0.3 cP
Benzine 0.50 cP
Chloroform@ 20°C 0.58 cP
Methyl alcohol@ 20°C 0.597 cP
Benzene @ 20°C 0.652 cP
Water @ 20°C 1.002 cP
Ethyl alcohol @ 20°C 1.2 cP
Mercury @ 20°C 1.554 cP
Benzyl ether @ 20°C 5.33 cP
Glycol @ 20°C( probably ethylene glycol) 19.9 cP
Linseed oil (Raw) 28 cP
Linseed oil (Boiled) 64 cP
Soya bean oil @ 20°C 69.3 cP
Corn oil 72 cP
Olive oil  @ 20°C 84.0 cP
Glass cooking 2450F 102 poise (100 cP)
Light machine oil @ 20°C 102 cP
Motor oil SAE 10 50-100 cP
 65 cP
Motor oil SAE 20 125 cP
Motor oil SAE 30 150-200 cP 200 cP
Motor oil SAE 40 250-500 cP 319cP
Motor oil SAE 50 540 cP
Heavy machine oil @ 20°C 233 cP
Caster oil @ 20°C 986 cP
Motor oil SAE 60 1,000 - 2000 cP 1,000cP
Glycerin @ 20°C 1,410 cP
1,490 cP
Motor oil SAE 70 1,600 cP
Pancake syrup @ 20°C 2,500 cP
maple syrup @25°C 3,200 cP
Venezuela’s Orinoco extra heavy oil reservoirs are about 53°C 1,500-3,000 cp
Honey 3,000cP
Honey @ 20°C 10,000 cP
Honey 2,000-3,000 cP
Blackstrap Molasses 5,000 - 10,000 cP
Molasses @25°C 8,700 cP
Soft end of glass working range (~1800F) 104 (10,000) poises
Chocolate syrup @ 20°C 25,000 cP
Hershey’s Chocolate Syrup 10,000-25000 cP
Ketchup @ 20°C 50,000 cP
Ketchup Heinz 50,000 - 70,000 cP
Ketchup @25°C 98,000 cP
Peanut butter 150,000-250,000 cP
250,000 cP
Corn Syrup 110,000 cP ??
Peanut butter @ 20°C 250,000 cP
Smooth Peanut butter @ 25°C 1.2x106 cP
Bitumen  Canada’s Athabasca reservoir sands are about 10-12% bitumen, at 11°C 1 x 106 cP
Crisco Shortening 1x106-2x106cP
1.2x106 cP
Glass Softening Point (sags of own weight) 107.6  (39,810,717) poises
Firm end of glass working range 108  (100,000,000) poises
Window putty 1x108 cP
Tar or pitch @ 20°C 3x1010 cP
Soda Glass  @ 575°C 1x1015 cP
Glass Annealing point 1013     (10,000,000,000,000) poises
Glass Strain Point (low end of annealing process) 1014.5 (316,227,766,016,838) poises
Earth upper mantle  3 to 10x1023 cP
Earth lower mantle  2 to 3x1025 cP

If there are two numbers listed above it refers to two difference sources of the information. None of the source measurements had any error number associated with them. 
[20°C = 66°F, 25°C = 75°F, 11°C = 51.8°F,  575°C = 1067°F]

A reader enlightens me with this interesting twist

A kind reader noticed (from an older version of this page) my failed attempt at explain viscosity (I tried to make the equation to mean a unit of power per unit of area - something it is not). My goal was to put viscosity in to an intuitive concept using an energetic approach to grasping the units.  My Idea was that viscosity could be seen as the ability to absorb power.

Thanks to our reader- in his own words which furthers the analogy

The classic definition for viscosity which you have put in the current version of your page is:
Viscosity = [Frictional Force x thickness x time] / [(Area of plate) x (distance)]

I would like to group this expression in a slight different manner, to show the physical meaning of viscosity:
Viscosity = [Frictional Force/Area of plate] / [(distance/time)/(thickness)]
The last part of the expression is the concept “speed variation for thickness unit” (actually the speed gradient in a direction perpendicular to flow).

So, the dynamic meaning of viscosity would be:
Viscosity is the force needed to apply on a plate of unit surface in direction of travel to move it to achieve a unitary speed variation for each unit thickness.
Or the same using differential analysis elements:
Viscosity is the force needed to apply on a plate of unit surface in direction to travel to move it to achieve a unit of speed gradient.
Now let as go to an energetic approach to viscosity.
Power (P) is force times speed.
Power = Force x speed
But frictional force is, according to dynamic formula:
Frictional force = [Viscosity x Area of plate] x [(distance/time)/(thickness)]
so, power is:
Power = speed [Viscosity x Area of plate] x [(distance/time)/(thickness)]
and the energetic expression of viscosity is now:
Viscosity = Power / {Area of plate x speed x [(distance/time)/(thickness)]}
Being the viscosity definition:
Viscosity is the power needed to apply on a plate of unit area to sustain a unit speed keeping a unitary variation of speed for each unit thickness.
Or using differential analysis expressions:
Viscosity is the power needed to apply on a plate of unit area to sustain a unit speed with a unit speed gradient.
So, using dimensional analysis:
“This is viscosity; a unit of power per unit of area, unit of speed and unit of speed gradient”

Hugo J. Gavarini

Was this Information Useful?

If you found this information useful - all I ask is to look at our home page and see if we have any products that might be of use to you or a colleague. Links are Appreciated . If you have something to add to this page please send it to the e-mail below.

Transtronics, Inc. Logo Registered trademark 3209 W.9th street
Lawrence, KS 66049
(785) 841 3089
(785) 841 0434
Bookmark this page


(C) Copyright 1994-2007, Transtronics, Inc. All rights reserved
Transtronics® is a registered trademark of Transtronics, Inc.


Contact Mike Firth