Glass Melting

(Reference: Battelle PNNL MST Handbook, U.S. Department of Energy, Pacific Northwest Laboratory)

Background Information: Mole Percent

When batching glasses, you need to perform calculations to figure out the weights of various chemicals needed. The concept of the "mole" needs to be developed before these calculations are done. The following background information will help you understand the mole.

Three ways of buying things commonly exist: volume (gallons of gasoline), number (dozen eggs), and weight (pounds of oranges). Volume is fairly good for liquids, and it is convenient, but it isn’t very reliable for solids because of such things as air gaps and irregularities. Also, volume changes with heat and pressure. Number is fine for regular things, but it is unfair to sell apples by the number since some are large and some are small. (Some of us have the same feelings about shoes and shirts too!) Most things are sold by weight, although these are usually pre-packaged, so we really buy them by the number of packages. Most of the things we buy by weight are "bulk" items. A few examples are bananas, grapes, candy at a candy store, coal, and nails.

One of the reasons nails are sold by weight is because small ones would be too boring and time consuming to count each time they are sold. If this were done, the price would also rise. In general, things are measured by weight to determine their number if things are too small to be conveniently handled. Two formulas that are used are:

These ideas and formulas are also used with chemicals. Atoms combine to form chemical compounds. For example, experiments enable us to know that two atoms of hydrogen combine with one atom of oxygen to form one molecule of water (H20). Unfortunately, atoms are far too small to be counted. Therefore, we use weight and the above formulas to determine numbers of atoms.

Because two atoms of hydrogen combine with one atom of oxygen to form one molecule of water, it follows that two dozen atoms of hydrogen combines with one dozen atoms of oxygen to form one dozen molecules of water. Also, 200 atoms of hydrogen combine with 100 atoms of oxygen to form 100 molecules of water. Atoms are far too small to see hundreds, millions, or even trillions of them; so, a new number called a "mole" is used with atoms. It is huge! A mole is 602,000,000,000,000,000,000,000. This can also be written: 6.02 x 1023.

 

Therefore, two moles of hydrogen atoms combine with one mole of oxygen atoms to make one mole of water molecules. Mole is a number that works just like "dozen" but is much larger. The mole seems like a weird number, but is was selected because it works with the atomic weights that are found on the periodic table. For example, carbon has an atomic weight of 12.0, and one mole of carbon atoms has a weight of 12.0 g. It so happens that one mole of any element equals its molecular weight.

To determine the weight of molecules in a material or a chemical, each element that is part of the molecule must be considered. Just as 1 dozen watermelons does not weigh the same as 1 dozen doughnuts, so 1 mole of carbon atoms does not weigh the same as 1 mole of oxygen atoms. They are equal in number, but not in weight. To determine how much a mole of atoms of a chemical weighs, we add up the atomic weights of all of the atoms in the formula for the chemical. For example, sodium hydroxide (NaOH) and ammonia (NH3):

Chemical Element Atomic Wt. Number of Atoms Total Wt.
NaOH

Na

23.0

1

23.0

 

O

16.0

1

16.0

 

H

1.0

1

1.0

Wt. of 1 mole NaOH =

40.0 g
NH3

H

14.0

1

14.0

 

H

1

3

3.0

Wt. of 1 mole NH3 =

17.O g

Therefore, one mole of NaOH molecules weighs 40.0 g, and one mole of NH3 molecules weighs 17.0 g, although there are equal numbers of each chemical.

Remember, a mole is just a term to represent a very large number. If we know the type of chemical, we can use the periodic table to determine how much a mole of that chemical weighs. Therefore, moles allow us to use weight to determine numbers of atoms or molecules.

In the glass batching lab, the concept of the mole will be used to determine the amount of each chemical to add to the glass. The calculations are explained in the following section.

Loss on Ignition

In this lab, almost 140 g of source chemicals are required to produce 100 g of glass. When you melt your glass, check to see how close you come to 100 g, and explain any differences. This loss of weight when melting material is called "loss on ignition" and must be accounted for when preparing the glass.

 

You may want to check to see how this loss on ignition works by performing a simple decomposition experiment. Gently heat a known amount of boric acid (H3B03) or Twenty Mule Team Borax (Na2B4O7 10H20) above its decomposition temperature, and then re-weigh the material after it cools. Compare the loss on ignition you measure to the one you calculate. If there are differences they may be caused by extra moisture in the sample, impurities in the sample, or vaporization of the sample. In most cases, the differences will be minor.

Activity Standard Glass Batching Calculations

Student Learning Objective

At the end of the activity students will be able to:

Materials

Procedure

Note: This procedure takes you through the entire process for calculating a glass composition. As you become familiar with these calculations, you will be able to quickly extract parts of these calculations to use in determining a glass composition. Be patient, it may take some time to understand all the concepts presented. After batching a number of glasses you will become familiar with these calculations.

1. The desired glass composition to be produced must be expressed as a mole fraction of each constituent. For example, a simple borosilicate glass could be expressed as Na2O B2O3 2 Si02. This chemical formula simply states that glass formers will exist in the glass in the ratio of one mole of Na2O (sodium oxide), to one mole of B203 (boron oxide), to two moles of 5 Si02 (silicon dioxide or silica).

2. Determine from the periodic table the weight of one mole of each of the oxide components of the glass expressed as grams per mole (grams/mole). This is a process for obtaining the molecular weight of a compound or chemical. For example, in determining the molecular weight of B2O3, we find the molecular weight of boron to be 10.81 g, and the molecular weight of oxygen to be 15.99 g. The weight of one mole of B2O3 is equal to 2(1 0.81) + 3(15.99), which is 69.62 g/mole. The calculations for all three glass components are shown below.

Na2O Na = 22.99 g/mole, 0 = 15.99 g/mole

Na2O = 2(22.99) + 15.99 = 61.98 g/mole

B2O3 B = 10.81 g/mole, 0 = 15.99 9/mole

B2O3 = 2(10.81) + 3(15.99) = 69.62 g/mole

SiO2 Si = 28.09 g/mole, 0 =15.99 g/mole

SiO2 = 28.09 + 2(15.99) = 60.08 g/mole

3. Determine the total molecular weight of the Na2O B2O3 2SiO2 glass by summing the weights contributed by each glass component.

Na2O 61.98 g
B2O3 69.62 g
2 SiO2 2(60.08) = 120.16 g
Total 251.76 g

4. Normalize each molecular weight fraction to 100 to determine weight percent. See below.

Na2O (61.98)/(251.76) x (100) = 24.62 weight percent
B2O3 (69.62)/ (251.76) x (100) = 27.65 weight percent
2 SiO2 (120.16)/ (251.76) x (100) = 47.73 weight percent

 

5. The sum of the weight percentages for all glass constituents must equal 100%. This is a good double check of the calculations.

6. Many raw materials are available as compounds that decompose to the desired oxide upon heating. Compounds such as Na2O and B2O3 unstable in air and so are almost impossible to obtain as pure compounds. Na2O is purchased as Na2CO3 (sodium carbonate. In the glass-making process, the Na2CO3 decomposes to form the desired Na2O. Because we will start with Na2CO3, this is called a "source chemical." Listed below are our glass components, their sources, and changes that occur:

Source
Chemical

Source Chemical
Formula

Glass
Component


+ Off Gas
Sodium carbonate Na2CO3 Na2O + CO2
Boric acid 2 H3B03 B203 + 3H20
Silica Si02 Si02  

We need to determine what mass of the source chemical is needed to produce one gram of component. This is our "conversion factor." Formula masses are used to determine this for each component.

a) Na2O Compound Element Mass Atoms Total Mass
Na2O Na 22.99 2 45.98
0 16.00 1 16.00

Mass of 1 mole =

61.98 g
Na2CO3 Na 22.99 2 45.98
C 12.00 1 12.00
O 16.00 3 48.00

Mass of 1 mole =

105.98 g

Because l mole of Na2CO3 produces l mole of Na2O, the ratio is:

 

Therefore, 1.710 g of Na2CO3 will produce 1.00 g of Na2O. This is our conversion factor.

b) B203 Compound Element Mass Atoms Total Mass
B203 B 10.81 2 21.62
0 16.00 3 48.00

Mass of 1 mole =

69.62 g
H3B03 H 1.01 3 3.03
B 10.81 1 10.81
O 16.00 3 48.00

The B203 component contains two boron (B) atoms, and the source contains only one B atom. Therefore, we need twice as much source. The ratio is:

Therefore 1.7769 of H3B03 is needed to produce 1 g of B203. This is our conversion factor for this chemical.

c) Si02 Because we are using Si02 as our source, the ratio:

Therefore, 1.00 is the conversion factor in this case. Note: no decomposition takes place with Si02.

7. We will now go through a procedure to summarize our previous steps.

a) Divide a sheet of paper into 5 vertical columns with the following headings:

Glass
Component
Weight % Source
Chemical
Conversion
Factor
Amount Needed
(for 100 g)

b) Fill in the component column with the compounds from step 1.

c) Fill in the weight percent column with the values calculated in step 3.

d) In the "source" column, place the formula of the actual chemical to be used in batching as noted in step 6.

e) For each source chemical, place the conversion factor that was calculated in step

f) Multiply the number in the weight percent column by the corresponding conversion factor to calculate the amount of source chemical needed to make 100 g of glass. Place this number in the "Amount Needed" column. A completed work-up sheet is provided below for batching 100 g of Na2O Na2O 2 Si02 glass:

Glass
Component

Weight %

Source
Chemical

Conversion
Factor

Amount Needed
(for 100 g)

Na2O 24.62   1.710 42.10 g
B203 27.65 H3B03 1.776 49.11 g
Si02 47.73 Si02 1.000 47.73 g
     

Total

138.94 g

 

Activity: Standard Glass Batching

Student Learning Objectives

At the end of the activity the student will be able to:

Materials

Equipment

Top-loading electronic balance with accuracy of 0.1 g

 

Procedure

1. Prepare a glass recipe workup sheet listing your source chemicals and relative amounts (generally expressed in grams) of each component to be used in the glass.

2. Collect all source chemicals into a common area (usually somewhere near the balance); clean area to avoid contamination of source chemicals.

3. Using a permanent marker, record glass oxide composition, weight percent, and your initials on the plastic bag.

4. Inflate the plastic bag to check for defects or leaks. Place the plastic bag inside the ice cream container. Replace lid.

5. Tare the balance, weigh the empty ice cream container, and record the weight on the container. [Tare means return balance to zero.]

6. Place weigh boat onto balance. Tare balance.

7. Carefully clean spatula or spoon with water or ethanol, then dry it. Be very careful not to contaminate chemical sources with other chemicals or laboratory grit.

8. Weigh out each of the source chemicals, one at a time. Transfer from weigh boat to the ice cream container, and check off the weighed chemical on the workup sheet. Note: Accuracy and cleanliness are important.

Caution: Avoid creating dust as the chemicals are being emptied into the collection container. Fine particles of any material are a health hazard.

9. When all constituents have been weighed out, a gross weight is then taken to ensure that no major constituent was omitted.

a. Obtain gross weight by 1) removing weigh boat from balance, 2) tare balance, and 3) weigh ice cream container with added chemicals.

b. Subtract tare weight of original empty ice cream container (step 3) from gross weight (step 6.) This weight should equal the total batch weight from the batch workup sheet. A deviation of 0.5 percent is acceptable.

10. The powder is now ready to be blended to obtain a homogenous mixture. Gently stir, avoiding making dust, to develop a fine uniform mixture free of lumps. The best method for mixing is to seal the open end of the plastic bag with air trapped inside the bag. Shake the chemicals for several minutes. With your fingers crush any lumps of chemicals, and reshake the batch. This process is known as "shake and bake."

Extension Activities

1. Discuss why a mixture of carbonates and oxides is used. Look up the melting points and decomposition temperatures for the various chemicals used (i.e., CRC Handbook; the chemistry teacher will have reference books like this). To get high melting point compounds [i.e., silica (Si02) or alumina (Al203)] into solution at much lower temperatures, glass modifiers such as sodium oxide (Na2O), lithium oxide (Li2O), and calcium oxide (CaO) are used. These chemicals have a relatively low melting point and are very corrosive in solution—especially a molten solution. Boron oxide (B203) is a glass network former as are silica and alumina. These chemicals form the network in the glass and keep the modifiers "locked up" or chemically stable in the glass.

2. Note that mixtures melt at lower temperatures than the pure compounds. Compare your glasses to more common mixtures like adding salt to ice water to lower its freezing point (i.e., making ice cream and clearing icy sidewalks).

Glass Melting - Instructor Notes

Reliability

This experiment works well. The quality of glass produced, though, depends on the formula, furnace temperature, and time at melt temperature.

Estimated Time for Activity

Two class periods.

Teacher Tips

1. Preheat furnace to 10500C. Do not exceed limit on furnace temperature.

2. Hot plate should be set on "high" and the annealing oven at 5000C. If the glass sticks to the hot plate or stainless steel, lower the hot plate temperature to "medium." This usually alleviates the sticking problem.

3. A good source for crucibles is DFC Ceramics (see Vendor List in Appendix). When ordering, ask that the crucibles be shipped UPS; this is less expensive than freight.

4. If you do not have a ceramic crayon, you can mark the crucible using a small brush and an iron salt or an iron oxide solution. Write on the crucible with the solution, and allow it to dry.

5. Stress to students that overfilling the crucible (more than 1/2 full) is a problem. As the chemicals heat and release gases, foaming will occur. It is often wise to check the melt about 5 mm after it is placed in the furnace to see if foaming is a problem. This is especially true if the students are trying different ratios or formulas of glass that they are not familiar with and do not know how much foaming to expect. Remove the crucible from the furnace if foaming is excessive. Start the melting over with a new crucible and even less of a chemical batch so the foam will be contained in the crucible. If foaming continues to be a problem, check the chemical formulation; one of the chemicals may have too much water in it and need to be pre-treated to dehydrate it before further use.

6. First pour powder into a glass beaker. If students pour powder directly from a plastic bag, heat from the crucible melts the bag.

7. Glass may soak at 10500C overnight to get good mixing but be sure your furnace maintains a stable temperature.

8. Students pouring glass for the first time are nervous. They often lift the pouring glass stream upwards, and as a result, the viscous glass puddle is pulled off the pour plate.

9. Usually, glass will have some bubbles in it. These bubbles are usually small and are remains from the foaming stage (decomposition of chemicals). The glass industry uses many techniques to remove bubbles from the molten glass. The students’ best technique will be time—lots of melting time if bubbles are not wanted.

10. Watch students as they cut the glass streamer. Sometimes they get burned, especially if scissors are too small. Cut the glass streamer close to the crucible to prevent hot "strings" from developing.

11. If spatula for transferring glass bars is not preheated on the hot plate, the bars will often crack. Keep the spatula hot until the moment the glass is to be transferred to the annealing furnace, then move the glass quickly, but safely.

Safety

1. Hot and cold glass are the same color, beware! Move hand slowly over glass to determine if it is hot.

2. Students must wear safety glasses.

3. Have students wear gloves when handling hot material.

4. Have students remove metal articles from their persons, especially rings. These materials transfer heat quickly.

5. Have students practice before they do the actual glass pour, using tongs and moving crucibles while they are cool.

6. Watch out for cracked crucibles, they may break if excessive force is used while moving or pouring.

Disposal

1. Follow school regulations for normal broken glass disposal.

 

Activity: Glass Melting

Student Learning Objectives

At the end of the activity the student will be able to:

Equipment

Procedure:

Warning: Wear Safety Glasses

1. Pre-heat furnace to appropriate temperature (usually 1050 0C).

Note: Schedule time to accommodate pre-heat. It takes approximately 1 hour to heat the furnace from room temperature to 10500C.

If annealing, pre-heat hot plate, stainless-steel pour surface, spatula and annealing oven approximately 1 hour before pouring. Set up bar molds.

2. Use a ceramic crayon to label a crucible with your initials, class period, and common name of glass.

3. In the fume hood, fill the melting crucible 1/2 full with the blended glass powder. Place the remaining powder, if any, into a Pyrex beaker, and set it aside.

4. Carefully open furnace door. Using gloves and tongs, transfer the crucible plus powder to the melting furnace. If oven is too hot, have your partner shield door with a ceramic heat shield. Close the furnace door. Allow glass powder to soak heat at temperature for approximately 20 min.

5. Remove the crucible and observe the melt. Powder should be molten with a viscosity of approximately 100 poise (consistency of honey). If it is not molten, increase set point temperature by 500C and repeat step 4. This step should be repeated as many times as necessary until the powder melts and resulting glass has viscosity near 100 poise or the furnace temperature capacity is reached.

Caution: Do not exceed furnace temperature limit.

6. Remove the crucible from the furnace and carefully pour the blended powder from the Pyrex beaker onto the top of the molten glass until the crucible is 2/3 full. Replace crucible in furnace. Several powder additions may be required before all glass is in the crucible.

7. Soak for 1/2 hour at 10500C. The melt soak time begins after the last addition of dry chemicals to the melt.

8. At 30-min intervals, stir the melt to ensure homogeneity by removing the crucible containing the glass and, while holding the crucible with metal tongs, mechanically stir the melt using a clean, 1/4-in. stainless-steel rod. This step can be skipped if care was taken in diligently mixing the chemicals. Note: Bubbling and foaming during the initial part of the melt also aid the mixing of the batch.

Pour Procedure

10. Using gloves, safety glasses, and tongs, remove the crucible from the furnace and either 1) air quench or 2) pour glass bars for annealing, according to the following steps:

a. Air quench - pour the molten material quickly onto a stainless steel pour plate. You may need to cut the glass from the crucible using scissors. Allow the material to cool until the glass surface is no longer dented by a slight tap of a metal spatula. Slide glass off pour plate into a stainless-steel beaker to contain flying particles produced when glass fractures upon cooling.

 

Figure 6.4. Glass Making

Caution: The stainless-steel beaker may become quite hot.

Note: Set hot plate on high (~3000C) and annealing oven at 5000C.

Safety Precaution: Heat-resistant gloves, reflective face shield, and safety glasses must be used when handling the molten glass!

b. Glass bars - Remove molten glass from furnace, and pour into heated bar molds as quickly as possible (see Figure 6.4). Allow this material to cool until top surface of bar is no longer dented by a slight tap from metal spatula. Dismantle bar mold rapidly, and transfer the bar to the annealing oven using a heated spatula. Soak at annealing temperature for 2 hours, then turn the oven off and oven-cool to room temperature. Do not open furnace until it has completely cooled; otherwise, the annealing process is disrupted and the 2-hour annealing must begin again. Sometimes the glass will crack or shatter if the annealing process is disrupted.

Note: This experiment may be interrupted or stopped at many places, which allows students to do the work over several days. Use caution, however, when allowing the glass to soak for extended periods (i.e., 4 hours) in the furnace. This will cause the crucible to erode. Moreover, certain chemicals such as calcium oxide or large amounts of sodium oxide can cause the crucible to erode in less than 30 mm.

 

Checking Annealed Glass

11. If the glass is clear and has been annealed, the glass can be checked for stresses by using two pieces of polaroid film. Sandwich the piece of glass between the two layers of polarized film, and hold the assembly so that direct light from an overhead projector or a fluorescent lamp passes through the materials. Rotate one of the polarized films 90 C so the light waves passing through the assembly are altered. Stresses in the glass will appear as reddish bands. In an unannealed or poorly annealed glass, the stress lines will be thin and numerous. In a glass partially annealed, and stress almost totally relieved, the bands will be broad and have almost no color. In a glass fully annealed, no lines or red bands will be observed.

12. For glass that did not anneal well, place the glass back into a cool furnace, turn it on, and allow it to heat 250C higher than previously annealed. Let it anneal for 3 more hours, then let it cool down in the furnace over night. Check for stress lines using polarized film the following day.