The Melting Points of Alloys

Muguet Cox

Student Projects, Metal Making

 

Introduction

The definition of an alloy is a mixture of two or more metals (or a metal and a non-metal fused together) dissolved in each other when molten. Throughout the History of Metal making, alloys have been tested and manipulated to form weapons, statues, coins, and jewelry. The Bronze Age came before the Iron Age due to the technology at the time with reaching high temperatures. Copper (Cu) was first discovered in its elemental state near the surace of the earth in 8000 BCE. About five thousand years after that in 3500 BCE, someone discovered that tin could be added to molten copper to produce bronze. This allowed for larger statues because the melting temperature of bronze was relatively lower than that of copper and the casting ability of bronze was much easier than that of copper. The Iron Age followed in 1500 BCE and in 900 BCE someone found that mixing zinc with molten copper would create the alloy brass with the advantage of its gold color and easy casting ability as well due to a lower melting point than pure copper. For this project, I decided to test the temperatures at the melting points of metals and alloys to better understand why it is better for an artist to work with combined metals rather than a single metal alone in the casting process.

Background

My project is comprised of four basic experiments with the intention of melting metals and alloys and discovering the temperatures at the melting points of the metals. The first experiment deals with finding the relative melting points of some metals and solders. The second experiment dealt solely with the melting points of solders. The third experiment allowed me to understand how two metals can converge into an alloy with a copper penny and zinc to turn the penny into brass. With the fourth and last experiment, I found the estimated melting points of some metals and alloys and converted the degrees in fahrenheit to centigrade. For enhancements, I attempted at casting two pendants and I also made two small sculptures using the solder alloys.

Procedure

Experiment #1 and 2- Which melt at a faster rate?

Materials- Bismuth (Bi), Lead (Pb), Tin (Sn), 50:50 (Sn:Pb) solder, 60:40 solder, 63:37 solder, hot plate, metal lid, gloves, goggles

I placed 5g of granular Bi, Pb, and Sn onto the hot plate after allowing it to sufficiently heat up to 500 degrees centigrade/ 932 degrees fahrenheit. I found that the Tin melted first followed by Bismuth and then Lead. With the solders, I let a different hot plate heat up to 400 degrees centigrade/ 752 degrees fahrenheit under the hood in the lab. I then placed 5g of each of the Sn:Pb solders onto the metal lid and timed how fast it took for each of the solders to begin to melt. I varied the amount of solder and tested the process again with 10g of each slder on the metal lid on the hot plate at 400 degrees centigrade.

Experiment #3- Zinc, Granular/The preparation of 'gold'

Materials- granular zinc (0.5 grams), 6 copper pennies dated 1982 or earlier (pennies before 1982 were solid copper and weighed more than the pennies produced today), 3 Molar sodium hydroxide solution, 3 grams sodium chloride, 15 mL vinegar, distilled water, two 100 mL beakers, evaporating dish, ring stand and ring, bunsen burner, matches, tongs, 50 mL graduated cylinder, gloves, goggles

I mixed the 3g of sodium chloride(measured on a scale) with 15 mLof vinegar(measured in graduated cylinder), in the 100mL beaker. I placed all 6 pennies into the solution to clean them until they were shiny. The, I removed the pennies from the solution and washed them in distilled water and making sure not to touch the pennies with my hands to avoid excess oils that might interfere with a reaction. I mixed 0.5g of granular zinc and 25mL of 3 Molar sodium hydroxide solution in an evaporating dish. Then, I placed the dish onto the ring stand over the bunsen burner and stirred the solution in the dish until steaming. Using the tongs, I placed 3 of the cleaned pennies into the steaming solution over the burner until they were completely covered with sodium zincate causing them to turn silver. I removed the silver pennies and diped them into distilled water to cool. After they had cooled, I used the tongs to hold two of the pennies over the bunsen burner flame and watched the silver color turn to gold, then immediately put the pennies into the beaker of distilled water.

Experiment #4- Melting Point Temperatures and conversion

Materials- calculator, controlled input furnace, Sn, Bi, Pb, Zn, Cu, Brass, Wood's alloy (Bismuth based), goggles, tongs, evaporating dishes.

For this experiment I wanted to find the melting point temperatures for each of the metals or aloys listed above. I fist looked in the Handbook of Chemistry and Physics to find the exact melting points and discovered that Pb begins to melt at 327.502 degrees centigrade, Zn at 419.58 degrees centigrade, Cu at 1083.4 degrees centigrade, Bi at 271.3 degrees centigrade, and Sn at 231.89 degrees centigrade. I placed 5g of each metal along with Brass and Wood's alloy into the furnace one at a time from lowest melting point to highest and fuond the estimated point melting points. The furnace went from 0-2000 degrees centigrade and I had to convert all of the results that I had using a conversion table. To convert centigrade to fahrenheit I had to multiply the number by 1.8 and then add 32. To convert from fahrenheit to centigrade I had to subtract 32 and multiply by 0.55. Also, for the Wood's alloy, I was informed that it would melt in boiling water and was the alloy used to set off fire sprinklers and is one of the most heat sensitive alloys. Therefore, i also boiled hot distilled water on a hot plate and dropped 5g of granular Wood's alloy into the beaker to see if it would melt as predicted.

How to insert a scanned image: The best way is probably to use a table (see Observations below) and then insert the images into the table, clicking on the cell in the table where the image is to go and then using Insert -> Image. However, you can just Insert an image wherever you want. It is best to make the image the size you want before you insert it into Dreamweaver, i.e., in Photoshop determine the size of each image.

 

Observations and Data

Experiment #1 and 2- With the metal melting I found that Sn melted first followed by Bi and the Pb. The handbook states that Sn begins melting at 231.89 degrees centigrade, Bi at 271.3 degrees centigrade, and Pb at 419.58 degrees centigrade. therefore I found the test to be accurate with the results that I had with relative melting.

For the Sn:Pb solders, I found that the less Lead in the alloy, the faster it will melt. This is due to the proportions of both Pb and Sn. The melting point of Sn is almost 200 degrees centigrade lower than the melting point of Pb. Therefore, the less Lead, the faster it will melt.

Sn:Pb solder (5g) Temperature of hot plate (degrees centigrade) Time taken for alloy to melt
50:50 400 3:05:40
60:40 400 2:46:23
63:37 400 2:09:15

The table shows that the solder containing less Pb melts faster that the other two.

Experiment #3-

In this experiment the reation of sodium zincate was formed when the elemental zinc was heated with the sodium hydroxide solution.

Zn(s) + 2NaOH(aq) --> Na2ZnO2(aq) + H2(g)

When the copper penny was added to the solution, the zincate ions, Zn(O2)2- , migrate to copper where they are decomposed and reduced to metallic Zn. The silver color produced is due to the metallic zinc coating the penny. When the zinc covered penny is exposed to heat, it will turn to brass.

Observations The pennies became shiny in a matter of seconds after placed in the vinegar and sodium chloride solution and swished around in the beaker. It took about one minute for them to turn silver in color after placed in the sodium hydroxide and zinc solution over the burner. Then, it took a matter of secods for the pennies to turn to brass while held over the flame.

 

Experiment #4-

I was unable to melt Fe because the furnace only went up to 2000 degrees fahrenheit and in order to melt Iron I would have had to use a heating device capable of reaching 2795 degrees F/ 1535 degrees C. When melting the copper and brass I noticed a change in color due to the heat. I found that it was much easier to work with an alloy than a solid metal. Because Crass is made up of Copper and Zinc, the lower melting point of the Zn dilutes the molten combination in weight and alloys the Brass to melt faster than just Copper by itself. The Wood's alloy did indeed melt in boiling water almost instantly as predicted.

Metals and Alloys Estimated Temperature in degrees Fahrenheit Temperature in degrees Centigrade
Sn 500 260
Bi 550 287.77
Pb 700 371.1
Zn 800 426.66
Cu 1800 982.22
Brass 1650 898.88
Wood's Alloy 160 90

Click on thumbs below to see full images.

stirring the pennies in the sodium hydroxide/zinc solution

the different phases of the pennies the melted solders, 63:37(far left). 60:40(middle), 50:50(far right) from the second experiment

preparing a wax mold, heating the nail tool

carving the wax mold heating the brass in the controlled furnace
molten Pb ceating plaster/work station poured brass in the plaster mold
molten wood's alloy in the plaster mold melting wood's alloy in boiling water  

Enhancements - For my sculptures I used flowering wire and a soldering iron to melt both the 50:50 Sn:Pb solder and the 63:37 solder. the sculpture that I made with the 63:37 solder took less time to make than the 50:50 due to the lower melting point and the lower concentration of Pb within the alloy. For the casting I prepared two plaster of paris (calcium sulfate and water) molds in plastic dishes. I then placed the gilf wax mold of a pendant into the wet plaster and allowed the mold to dry over night. The next day I heated the Brass and the Wood's alloy to make two pendants and found it easier to work with the Wood's alloy than the Brass. The Brass splattered on all over the mold because of the high temperature.

Conclusions

In conclusion, I found that metals have higher melting points than alloys because they are not combined ith other metals to bring the temperature of the melting point down. For an artist, it would be much easier to manipulate an alloy rather than a metal because of the lower metling points. For example, it would be much easier to cast a brass sculpture at 1650 degrees F than a copper sculpture at 1800 degrees F.

Other References

Carpenter, Sir Harold. Metals. Oxford University Press: London, 1939. pg. 208-300.

Zinc Granular, The Preparation of 'Gold' . Flinn Scientific, Inc.: Batavia Illinois, 1994.

Acknowledgements

My thanks goes out to Dr. Robert Bachman of the Chemistry Department who assisted me with this project by providing information on materials and how to go about working with the metals in the casting process.

Ms. Fitz provided me with several lab experiments to choose from.