Frequently Asked Questions

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Ceramic Shell

What is the purpose of the primary slurry?

The primary slurry has two main functions: to capture the detail of the wax pattern and to provide the surface that the alloy will be cast against.

What is the purpose of a backup slurry?

The backup slurry is used to create the bulk of the shell. It will provide the strength for the shell to survive the dewax and casting process. The backup shell coats will also determine the dimensional and heat transfer properties of the shell.

What is the recommended primary slurry formulation?

The primary slurry formulation is chosen based on the parts being cast, the alloy used and the process. A good starting point can be found on R&R's primary binder application instructions or by contacting R&R’s technical team at 800.253.4502.

How many primary coats will I need to use?

This depends on the slurry used, the surface requirements and the parts being dipped. In general, primaries containing high percentages of zircon are used for ferrous and other reactive alloys. One or two coats would be required with zircon stucco. For non-ferrous applications, slurries with lesser amounts or even no zircon could be used. The number of primary coats required is typically two; however, parts with a lot of detail and with critical geometry may require more primary coats to fill tight areas.

What refractory should I use?

This depends on the metal being cast. When casting a ferrous metal, a zircon slurry can be used for primary coats. In some cases, fused silica can be blended in as well. For non-ferrous metals, fused silica can be used in primary slurries, although even in these cases some zircon may be blended in as the density of the zircon will help the flow properties of the slurry. For backups, the typical choices are fused silica (Ranco-Sil™ fused silica) and aluminosilicates (Mulgrain 47, 60, 70).

What is the advantage of using fused silica as a backup refractory in my shell?

Fused silica has a low density (about 2.2 g/ml) and pound for pound will go a longer way. Shells made with fused silica are lighter. Fused silica is very rigid at casting temperatures and is not prone to shell bulging during casting. Fused silica is also much easier to knock out as it goes through a volume/phase transformation during the casting process.

What is the advantage to using aluminosilicates as a backup refractory in my shell?

The primary advantage of using aluminosilicates is that it is very abundant and is more economical than fused silica.

What is the benefit of having a two slurry system instead of using one type of slurry through the entire process?

The benefit of a two slurry system is that you can formulate each slurry to optimize the process of shell building. The goal of the primary slurry is to capture the detail of the pattern and provide a protective layer that the metal can be cast against with minimal reaction. Backup slurries are formulated to provide bulk and strength to the shell, typically consist of larger refractory particles and are formulated to have a lower viscosity.

What is the recommended process for shell coats (number of primaries, number of backups)?

This depends on many factors (part geometry, wax, shell system, etc.), so to give one answer would not be completely accurate. A typical process would be two primaries with four or more backup coats and a seal. The goal is to create a shell that will withstand the pressures that are applied during the dewax and casting process.

How much time should I allow for drying between coats?

There are four factors that impact the shell drying time. First is part geometry. A simple part will dry faster than a part with holes and passageways in any similar conditions. Next, the shell system used will have an impact. Some shell systems apply thinner layers that can be dried quicker. The last two factors are related to the drying environment, more specifically relative humidity and airflow. Lower humidity and increased airflow will speed drying. From a drying standpoint, the most bang for the buck is from increasing airflow.

How do I know when a shell is dry enough for another coat?

There are many ways to determine shell dryness. Some methods are more experience-based, like scratching the area and seeing if it is dry (a wet shell will be softer when scratched with a rigid object). There are drying indicators, like R&R's ReDip™ indicator that is one color when wet and one color when dry. Another method is to weigh the shell; once the shell stops losing weight, it is dry. Some foundries use a laser pyrometer to tell if the shells are dry. In this case, as the shells reach ambient temperature, then they are dry enough to coat.

What is the shelf life of my slurry?

This depends greatly on slurry maintenance and turnover. There is no set rule for shelf life. For primes, slurry life in “controlled” slurries typically ranges from three months to one year. With backup slurries, if turnover is adequate, the slurries will last indefinitely. The key to slurry life is a good slurry control program. For complete details on required tests and specifications for slurry parameters, refer to our Slurry Control Technical Tip.

What is the recommended process for burnout cycle?

A minimum burnout at 1600-1800°F for 1 hour after all organics have been burned out is suggested. Typically burnout for shells that have been dewaxed in an autoclave is longer than those who use a FlashFire Dewax Oven.

What is slurry control and why do I need to do it?

Slurry control is the process of running a series of tests on the slurry and comparing results to the specifications. Once this is done, action can be taken and additions will likely be required to bring the slurry parameters back into specifications. By keeping a slurry in control (i.e. in specifications) the slurry will perform at its best and will have the maximum slurry life.

How do I conduct the tests required to control a slurry?

There are many tests that can be run to control a slurry. Each of these tests is described in detail in our Slurry Control Technical Tip.

How do I correct my slurries once testing indicates they are out of specification?

The adjustments to bring a slurry back into control are easily calculated. For your convenience, automated Slurry Control Worksheets (Microsoft® Excel®) are available for download to calculate slurry parameters as well as slurry additions. These are used in conjunction with our Slurry Control Technical Tip.

How do I handle pH drift in my slurry?

If your pH is low, the addition of reagent grade ammonium hydroxide can be added at 0.5-1.0% by weight (150 ml/gallon of water). An alternative to that is using Triethanolamine (TEA) at 80 ml/gallon. This dilute solution should be used when the slurry requires normal water additions and the use of this dilute solution should begin when the slurry binder pH reaches 9.50 or below.

What does it mean if your pH is low?

It means that the colloidal is aging and becoming unstable. The colloidal is stabilized by -OH ions that repel the colloidal particles. Think of the pH as a measure of the OH ions in the binder. As the -OH ions decrease, so does the stability of the colloidal. This is a natural occurrence and it cannot be avoided, however, it can be slowed through proper slurry control and by making pH adjustments.

What does it mean if your binder solids test high?

This means that the silica particles within the binder are at too high of a concentration. When the concentration level of the colloidal particles increases beyond the point that the particles can repel each other, then the particles start to condense or agglomerate. This destabilization of the colloidal is known as “gelling”. This will affect the performance of the slurry and the quality of the shells that are produced with the slurry. Once a slurry begins to gel, it cannot be reversed and slurry life and performance will be diminished. For complete details on how to test for binder solids, see our Slurry Control Technical Tip.

What does it mean if your binder solids test low?

When the binder solids are too low, the colloidal within the slurry has been diluted too much and, while this will not affect life, it will impact the strength of the shells that are produced. There will be fewer colloidal bonds in the shell layers and the strength of the shell will be reduced.

What does it mean if your refractory solids are high? Low?

High refractory solids can make the shell brittle, causing hot tear defects and non-fill. Low refractory solids result in weak shells, shell leaks, run outs and bulge.

How do I measure the viscosity of a slurry?

The viscosity is measured using a flow cup typically known as a Zahn cup. There are various manufacturers of Zahn cups and each can provide different results, so be sure you always use the same cup type for consistency. Cups are designated by the size of the hole at the bottom of the flow cup. The smaller the number, the smaller the hole (a #2 cup has a smaller hole than a #4 cup and for the same slurry the #2 will give a higher reading). Viscosity is determined by the amount of time it takes for all of the slurry to exit the cup. A Zahn cup should be inserted into the slurry, filled and emptied to “wet out” the cup. The cup is reinserted into the slurry and filled. As the cup exits the slurry, a stopwatch is started. When the cup is empty, the stop watch is stopped and the viscosity is the time on the stop watch.

How do I know when to stop the stop watch when checking viscosity?

The endpoint of a viscosity test can be subjective. Sometimes there is a distinct endpoint when slurry no longer flows. Other times the endpoint is determined to be when the exit stream of slurry begins to break into droplets at 1 inch below the cup; and yet other times the endpoint is determined to be when the hole in the bottom of the cup is visible as the slurry exits when looking into the cup from the top. Any of these methods is acceptable as long as the method is performed consistently time and time again by all operators on all shifts.

Where can I get a viscosity cup?

R&R recommends the Zahn Signature series of cups. These cups are available by contacting R&R's customer service team at 800.253.4502.

What is MOR? Why is it important?

Modulus of Rupture (MOR) is for determining shell strength at green, hot and post fired states. Wax test bars are dipped using a determined shell sequence, then tested in an MOR unit. Shell strength is normally recorded as MOR because the amount of load strength to break a test specimen is recorded per area of the specimen at the break site. Theoretically, no matter how thick or wide the test bars may be, the MOR strength of identical ceramic material should remain the same. This data is important because it gives a glimpse as to how the shell system is going to perform through the process.

What is shell permeability? What does it tell you about your shell system?

Shell permeability is the measurement of a shell’s ability to allow fluids to pass through the material. In this case, the fluid is dry air. The higher the reading, the more permeable your shell. Testing shell permeability is important as air trapped in the shell cavity during casting can lead to gas and non-fill defects.

Does R&R test MOR and permeability?

Yes, we test both MOR and permeability.


Is the amount of water I use important to the result of my glass casting?

Yes, the amount of water used has great influence on the fluidity and working time of the mixed investment and on the strength of the resulting mold. Deviating too much from the recommended water/powder ratio can generate massive defects.

If too much water is used, the mold will be too weak to hold the molten glass. In this case, the glass can break through the mold and leak out onto the kiln. As casting size increases, this becomes even more important since the larger mass of molten glass places more stress on the mold material.

If too little water is used, the working time of the mold material will be too short which will not leave enough time to complete all of the mold making steps. This can result in loss of detail reproduction, trapped air and finning.

The R&R Glass-Cast mold materials come with a silica warning, what does that mean to me?

Crystalline silica is one component of R&R Glass-Cast mold materials. This portion of the formula is necessary to allow the mold material to reach appropriate firing temperatures. Crystalline silica has also been identified by the International Agency for the Research of Cancer (IARC) as a substance known to cause cancer in humans after prolonged exposure.

The warning provides data on the possible delayed lung injury effects of prolonged exposure to the dust generated from using R&R Glass-Cast mold materials. Precautions, such as face masks and dust collection systems, should be used to limit the amount of silica breathed while working with these mold materials. The Safety Data Sheet (SDS) provides detailed information.

Will fibers improve the strength of my mold? Will they affect the surface of the glass?

Similar to the way rebar improves the strength of concrete, fiber glass can improve the strength of a mold. While the investment is fluid, plaster flows all around the fibers, shielding them from the pattern itself. What results is a plaster barrier between any fibers and the casting. This barrier keeps the surface of the glass from being affected.

Will the temperature of my water affect the mold?

Not directly; however, temperature does affect the working time of the mold material and this can affect the mold indirectly. For every 10°F increase in temperature of the water and powder, the working time of the mold material decreases by approximately 1 minute. If the mold making steps are not completed within this shortened working time, the result will be a malformed mold.

Conversely, if temperatures are lowered, the working time is increased and there is a possibility that the water can separate from the powder before the mold material sets. This results in water-marking on the casting. R&R Glass-Cast 910 investment and R&R Glass-Cast 400 investment have special chemicals to prohibit this separation. R&R Glass-Cast 101 BANDUST investment and other investments do not.

Is a face coat necessary for using R&R Glass-Cast mold materials?

No, each of the R&R Glass-Cast mold materials can reproduce the finest detail without a face coat. During the mold making process, plaster rich water flows into the tiniest crevasses even though coarser refractory particles cannot. This plaster rich water will transform into gypsum and set hard. In a sense, a face coat is naturally formed during this process.

Rough castings can result if this plaster rich skin is somehow disturbed. A common mistake that leads to rough castings is excessive steam dewaxing, which erodes away this rich gypsum layer. Scraping the inside of the mold also will remove the gypsum-rich layer, resulting in in rough areas on the casting.

If using a face coat, the mold material may not bond consistently with this layer. This can cause inclusions in the casting and loss of detail in reproduction. Face coats often have different expansion characteristics than the rest of the mold materials as well. This results in cracking and shifting in the mold, leading to fins on the castings.


Can I use Astro-Vest investment to cast stones in place or for gold casting?

No, Astro-Vest is not recommended. For casting stones in place for gold, we recommend Solitaire investment, as it is enhanced to allow casting stones set in wax and provides casters with superior stone protection.

What is the maximum temperature for Astro-Vest investment burnout?

Phosphate bonded materials, like Astro-Vest investment, can withstand temperatures of 1800°F (982°C), but these temperatures are not normally required for most casting applications.

For what alloys do you recommend ADVANTAGE investment?

ADVANTAGE investment is recommended for casters of silver, brass and bronze jewelry pieces.

What are the application differences between ADVANTAGE investment and Ultra-Vest investment?

Both products are prepared in an identical manner.

If I alter the amount of water or binder being added to the investment, will the mold strength be altered?

Yes, if you reduce the amount of water or binder in a mix, you will create a stronger mold. The mold will dry faster, but the working time will not be affected. Remember that a stronger mold is harder to clean from the castings.

What is the difference between Solitaire investment and other R&R gypsum investments?

Although Solitaire investment is a gypsum-based material, it is designed specifically for stone in place casting. It contains different characteristics and ingredients than regular gypsum investments that protect the gemstones during the high temperatures of burnout and casting. In addition, Solitaire investment is not designed to quench out as easily as regular gypsum investments.

Why does Solitaire investment turn green when I mix it with water?

Coloring was added to the formulation as a safety measure. This allows you to confirm before casting that the proper investment is being used with stones. The color itself does not affect the performance of the investment or its measurable properties.

Can I steam dewax with Solitaire investment too?

Yes, then simply follow the recommended burnout cycles.

I cast gold pieces, with and without stones cast in place. Can I use the Solitaire investment for both types of castings?

You could, but this is not recommended as Solitaire investment does not have the same quenching characteristics that Ultra-Vest investment has. Therefore, you will likely have a difficult time trying to quench flasks cast in Solitaire investment.

What are your recommendations to remove Solitaire investment from the tree?

Molds containing stones are not water quenched, but rather allowed to air cool to ambient temperatures. High pressure water may be used with care to aid in investment removal.

What types of stones can be cast with Solitaire investment?

Hard, small jewelry grade stones (.25 carat and smaller) make up the bulk of the stones being cast in place. Jewelry grade stones can be synthetic as well as natural, but they need to be hard enough to withstand the high temperatures experienced at burnout and casting. Be advised that industrial grade stones, which usually include defects and inclusions (hence, they cannot be rated as jewelry grade) are never recommended to be cast in place. The inclusions can allow oxygen into the stones, resulting in damage (burning) during a normal cast in place process.

For which alloys is Ultra-Vest investment recommended?

Ultra-Vest investment is recommended for gold, silver, brass and bronze jewelry casting. Similar alloys are appropriate for this investment, provided the melting temperature of the alloys remains below 2200°F (1200°C).

What is the highest flask temperature you would recommend for Ultra-Vest investment?

Ultra-Vest investment molds will stay stable up to 1400°F (760°C). Normally, you would not have a need to get the flask this hot. Once the mold has been cured and fired, reduce the oven temperature to the desired casting temperature. If you ever have to delay casting and your oven is filled with burned out molds, hold the molds at the desired casting temperature. If you cannot complete the casting, do not drop the temperature below 400°F (205°C). Cast as soon as possible.

What investment is recommended to cast white gold (palladium)?

We recommend Ultra-Vest MAXX investment for white gold (palladium).

Is your investment hazardous?

Due to the presence of respirable particles (<10 microns) of crystalline silica in investments, these products do carry a respirable warning. It is extremely important to determine the level of exposure to which your operators are regularly subjected. The Safety Data Sheets (SDS) outline the personal protection guidelines for operators, based on the amount of exposure. Refer to the SDS for the product you are using and contact a health and safety expert with any questions you may have.

How long should I wait to quench out my gypsum molds?

Please consult your alloy supplier for the recommended quench time for the specific alloy you are casting.

Are there any uses for the spent (used) investment? Can I grind the material and add it back into fresh material at some small ratio?

Although there are probably a number of practical uses for the spent investment, we cannot recommend anything other than disposal. After casting, the investment may carry traces of heavy metals that can limit other potential uses. In addition, the thermal properties of the investment have been permanently altered by the burnout, casting and quench processes.

Do investment molds shrink?

Yes, but the amount of shrink is very small, around 1%. Keep in mind that the wax, metal and investment all shrink during some part of the casting process.

I have been advised to roll my investment drums and redistribute the powder before I use it. Is this required?

No, this is not required. However, a few casters roll the package simply to decompress settled material, making it easier to scoop out.

You recommend vacuuming until the investment rises and breaks. You say this should take less than a minute. If it doesn't occur in a minute’s time, should I keep the vacuum applied until it does?

If the investment doesn't rise and break quickly during the investment cycle, there is usually a problem with the vacuum. Vacuum pumps with old oil or leaks can cause the vacuum to pull less vacuum than a well-maintained unit. An oil change often makes a significant difference. You can check the system by placing water under vacuum. If you do not see the water boil rapidly, you should get the vacuum system checked. If the water boils and your investment does not, contact your investment manufacturer.

Is distilled or deionized water required, rather than tap water? What is the difference?

Distilled or deionized water is recommended because it is more consistent than tap water. Impurities in tap water can change the working and set time of an investment. With distilled or deionized water, the impurities are minimized so the investment properties are more repeatable drum to drum and batch to batch. All of R&R’s retained batch samples are evaluated using deionized water to ensure batch to batch consistency.

At times, I would like to lengthen or shorten the set time of the material I am investing. Do you have any suggestions?

In general, warmer water and/or warmer powder will speed up the setting time. Cold water and/or powder has the opposite effect.

Does a flask really have to set undisturbed for an hour or two before removing the sprue base? I would like to speed up my production cycle and remove the base sooner than an hour.

The investment does require at least an hour to set completely. Many customers see finning and other defects associated with removing the sprue base too early. If the base is removed before the investment has sufficient green strength, the investment is shifted and stressed when the base is removed. This can result in rough surfaces and other casting defects.

Is there any easy way to increase the strength of a mold without compromising quality?

Reducing the amount of water used will increase mold strength. Rather than using a 40/100 ratio for a gypsum investment, you may want to try 38/100. However, the increased mold strength will reduce the ability to remove investment after casting. The permeability of the mold will be reduced when less water is used.

Can I add water to the powder?

This method of mixing is not advised since it takes too long to completely wet out the powder. The material has a set working time and it is important to keep the mix time (3 minutes or less for a gypsum investment) as recommended so there is enough time to compete the entire process. Adding the powder to the water will allow you to mix thoroughly in the desired time.

Do I need to use the entire recommended working time for your investment like other manufacturers recommend?

Other manufacturers may advise using the entire working time as a means of eliminating water marking defects. Water marking is characterized as “rivers” of metal on the underside (as invested) of the casting. It is caused by water separating from the investment as it sets. Use of the entire working time is not a requirement to achieving good results with R&R investments. The working time is set to allow a large portion of the casting industry enough time to prepare investment and fill flasks. However, do make certain you mix the investment with water as long as recommended in the application instructions.

Are there any differences in investment powder preparation or use if I am using plastic, rather than wax, patterns?

No, there are no differences in investment powder preparation based on the patterns used.

How much investment should I have above the wax tree and around the sides of the flask?

Plan to cover the tree with at least 1" of investment and about 1/4" of investment between the wax and the flask walls. This thickness provides good strength for casting.

I've noticed that investments seem to heat up a bit while they are mixing. Why?

Gypsum is somewhat exothermic. The more material you mix, the warmer the mix will become. This is not a problem because the vacuum will cool the investment and you may even notice a cooler flask after the second vacuum step.

I have heard people refer to set time, pour time, working time and gloss off time when they speak about investments. Can you provide clear definition of these terms?

Let’s define them from the shortest to the longest time frames. Working time starts when you add the powder to the water and ends when the material starts to thicken. For R&R’s gypsum materials, this happens 8-9 minutes after the start of the mix. For our phosphates, this occurs in 4-5 minutes. Pour time starts with the powder to water addition and stops when the material is physically too thick to pour from one vessel to another. Gloss off also starts with the powder to water addition but ends when you can no longer see moisture on the top of the flask. Set time starts at the same point and is timed until a Vicat needle will no longer penetrate the material more than 1 mm deep. Our gypsum products have a set time less than 20 minutes.

How do I determine the pounds of investment needed for my flask?

Use the appropriate flask calculator for your investment; located under the Resources section of the R&R website.