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Natural Latex FAQ

Question: WHY HAS YOUR #80 FORMULA REMAINED THE SAME FOR SO MANY YEARS?

“If it ain’t broke, don’t fix it”.  This adage is true for the #80 formula.  Unlike Silicones and Urethanes, where the shore hardness must be variable, Natural Latex molding compound #80 dries to a very flexible rubber with a shore hardness that is superb for concrete and plaster.  If the #80 is vulcanized properly, several castings with polyester resin can be made.  The problem with formulating natural latex molding compounds is that variations complicate the
mold making process.  Natural Latex is also a difficult product to control in compounding.  If #80 is not compounded according to its time-tested formula, it will veer out of control.  Exacting quality control has made our Natural Latex Molding Compound #80 the standard of the industry.

Question: IF MY LATEX IS TOO THICK, MAY I USE TAP WATER TO DILUTE THE LATEX?

Tap water is not a very good thinner for latex since it may contain Calcium and Magnesium salts.  Even small amounts of the salts of Calcium and Magnesium have a tendency to coagulate the latex. Hard water must never be used.

Question: WHAT IS BEST TO USE TO THIN LATEX?

If you wish to thin out the latex, simply use distilled water or bottled water. Read the label carefully, several bottled waters contain calcium.

Question: HOW CAN I THICKEN THE LATEX TO FILL IN DEEP UNDERCUTS?

Use CECO Powder.  It will thicken the latex to a point where it can be applied with a trowel.  It will also strengthen the latex.  Only mix CECO Powder in backup coats after the first few coats have been applied.

Question: HOW LONG SHOULD I HEAT THE RUBBER TO COMPLETE
THE PROCESS OF VULCANIZATION?

Vulcanization is the combination of Latex with Sulfur in the presence of heat. (Remember Goodyear?) If a rubber is not vulcanized, it has poor keeping qualities and usually does not keep its original form, especially upon stretching. I would recommend 24 hours at 100-120 degrees farenheit.  Or, let it stay at room temperature for about 4 days before starting your fiberglass back up.

Question: WHAT EXACTLY IS VULCANIZATION?

If we avoid all academic discussions about Vulcanization, we can say that Vulcanization is named after Vulcan, the ancient Roman host of fire, and it is the combination of various ingredients with the rubber under controlled heat to create a flexible elastomer.


Rubber that is not vulcanized is usually hard under low temperature and very soft at high temperature. Un-vulcanized rubber is also "tacky", and when
stretched, deforms very easily. Un-vulcanized rubber shows "plasticity" as opposed to "elasticity".  Vulcanization corrects these defects while greatly
increasing the tensile strength of the rubber.

Question: WHY DOES LATEX HAVE AN AMMONIA SMELL?

Latex comes to us from the plantation before we begin compounding it, and it needs to be preserved for it's voyage.  Ammonia preserves latex and is also an anti-microbial. It's a long way from the plantation to our faciities on the east coast.

Question: LATEX SOMETIMES HAS A HEAVIER AMMONIA SMELL WHILE AT OTHER TIMES IT IS A LITTLE MILDER.WHY?

Holden's imports latex directly from several sources in the Far East. Sometimes the latex gets contaminated by bacteria. Since ammonia is used not only as a preservative, but also as a fungicide, the stronger smell indicates that more was needed to combat bacterial action. You may also notice a reduction in viscosity associated with the additional ammonia.

Question: DO YOU PREFER AMMONIATED LATEX OR "NO" AMMONIA LATEXES FOR MAKING MOLDS?

We prefer higher ammonia concentrated latex.  One of the basic reasons is that latex preserved with alkali has a marked tendency to be "sticky" and "tacky".  Another reason is that non-ammoniated latex has a tendency to absorb water so that evaporation is very slow.  This means long waits between coats.  Our #80 ammoniated latex is designed to coat about every hour, as opposed to the de-ammoniated latexes.

Question: HOW LONG DOES YOUR #80, #660, AND SIMILAR BRUSH-ON MOLDING COMPOUNDS LAST?

The shelf life of Natural Latex is anywhere from 6 months to a year.  It has optimum qualities till about 6 months and then slowly loses some of its mold making characteristics.  It takes about a year for it to lose almost all of its properties. If it separates into 2 layers, gentle stirring with a stick will bring it back.  If it looks like cottage cheese, it should not be used.

Question: IF LATEX "SEEMS" OK AFTER 6 MONTHS, WHAT SHOULD I USE IT FOR?

Use it for back-up coats.  If it is still brushable, you may use it, but for back-up coats only.

Question: HOW LONG SHOULD I WAIT BETWEEN COATS OF LATEX?

Our products, #80, #660, etc., are designed to be coated about every hour.  This is not a hard and fast rule and judgement is required.  You do not have to wait a few hours between coats.  However, avoid waiting more than 25 or 36 hours between coats.

Question: MY BRUSHES DRY WHEN I COAT LATEX ON MY MOLD.WHAT SHOULD I DO?

Keep several inexpensive brushes in thick,soapy water.  Use Ivory liquid soap or Murphy's oil soap, or something similar.  Remember that latex dries by the evaporation of water, so, naturally, your brushes would get all "gummed" up.  Remember to shake your brushes out well before you begin coating.

After your brush gets stuck, place it back in the soapy water and switch to another brush.  Comb your previous brush out with a fork or dog comb.  If you still have dried rubber in your brush, soaking the brush in lacquer thinner will help a lot.

Question: WHAT DO I DO IF I GET A BUBBLE OR A BLISTER WHEN
I AM BRUSHING ON THE #80 LATEX COMPOUND?

Burst the bubble with a pin or use a syringe to withdraw the air.  When burshing, brush up and down and left to right.  This helps burst the bubbles.  Then simply continue brushing.

Question: WHAT IS THE “ BRUSH ON METHOD” FOR MAKING MOLDS?

The “Brush on Method” for making molds has always been associated with natural latex.  It is the process of applying the rubber, like thick paint, with a brush. The mold is built up through multiple layers applied with a brush on top of each other. Urethanes and Silicones are also referred to as “Rubber”. Remember, though, only natural latex, when brushed layer upon layer, has the superior flexibility needed for making concrete and plaster reproductions.

Question:WHO DISCOVERED THAT NATURAL LATEX ADHERES TO ITSELF?

For that again we return to the practices of ancient civilization.  The natives of Central America noticed that when they cut fresh natural rubber (milk of weeping trees), the cut ends adhered to themselves.  The same principle is applied to mold-making.  Freshly coated latex accepts another coat of latex so it is possible to build up your layers. Don’t forget, though, about 48 hours is the limit before adhesion problems begin.

Question:WHEN DID THE PRODUCTION OF HOLLOW OBJECTS ACTUALLY BEGIN?

Since we are speaking about products to produce hollow objects, the answer to you question can be found in ancient civilization.  When early explorers landed in the “New World”, they noticed that the natives poured the milk of “weeping trees” into clay formers to build up layers of this milk of the weeping tree.  The natives then broke the formers and retained the “rulling” material that was formed.  These formers were actually used to produce water bottles or to waterproof cloth. In the 1800’s, the industry turned to Plaster of Paris to make hollow objects out of rubber.

The 1880’s added another dimension to the production of hollow objects by using fillers to harden the latex. Holden's compounds HX-200 casting compound and manufactures a filler, so that you may vary the degree of hardness.

Question: WHAT CAN I DO TO PRODUCE HARD HOLLOW OBJECTS?

Hard, hollow objects can be produced by using HX-200 in combination if 3 parts of HX-FILLER, which will produce hard hollow objects when casting into a plaster of paris mold, using the slush casting method.

Question: HOW DO YOU REPAIR A NATURAL LATEX MOLD WHEN IT TEARS?

When a tear develops in a natural latex mold, it is very difficult to repair it completely for long runs.  It may only be repaired temporarily.  The best method we found is to clean the area around the tear with acetone. Then soak some gauze in natural latex, squeeze out the gauze and place it in the tear. Hold it together until it sets a little and then trim the gauze. Then put a number of coats of latex over the gauze.  Let it vulcanize.  The repair should hold for another ten castings, or so.