Basic Plaster Craft Instructions
Part I - Mold Preparation
By Bob Sherman
Plaster craft is a fun hobby which allows you to turn a few pennies worth of materials into beautiful wall hangings and sculptures. Although there are several techniques used for plaster casting, in this tutorial series we will be discussing the simplest method - making flat back plaster castings in open top molds. Most common plaster molds are this style, but chocolate molds also work well for plaster. Avoid two piece or 3D molds until you have some experience.
PLEASE NOTE!! - Plaster crafting is fairly safe if you observe these safety rules when mixing plaster. Once hardened there is little or no hazard from handling plaster items:
- Plaster is very alkaline and and exposure to high dust levels may irritate the skin, eyes, nose, throat, or upper respiratory tract. Wear a dust mask, eye protection, and rubber gloves when mixing plaster.
- Do not wear contact lenses when working with plaster.
- Plaster generates a lot of heat when setting - never use plaster to make casts of body parts.
- Keep away from children or pets.
- Other safety precautions may apply depending on the plaster product you use - read the directions.
Plaster is used throughout this article to describe casting products containing mostly plaster of paris. The two most common are plaster and hydrocal.
- Plaster is generally 100% plaster of paris and is called a variety of names depending on the manufacturer. These include: Plaster Of Paris, Casting Plaster, or just plain Plaster.
- Hydrocal is a mixture of plaster of paris and portland cement and is called a variety of names depending on the manufacturer. These include Hydrocal, Hydrostone, and Ultracal. Typically hydrocals are 3 times stronger than plaster and more dimensionally stable. For castings with thin areas hydrocals are a good choice.
Plaster Casting Supplies
The following plaster craft supplies will be needed to follow along with this series of articles. Clicking on the item name will bring you to that item's page with a full description and ordering information.
Please Note: We are discontinuing plaster crafting products but have left these instructions up as a public service.The links below may may be non functional by the time you read this.
|Plaster Mold||A medium sized mold of your choice. A suitable chocolate mold may be used if you prefer. Chocolate molds are a good alternative to plaster molds since they are inexpensive and available in a huge variety.|
|Mixing Bowl||A flexible mixing bowl simplifies cleanup, but you may use any plastic bowl.|
|Plaster Blender||This requires a power drill to use, but makes blending the plaster much easier. If you don't have a power drill, a potato masher may be used to mix the plaster manually. For very small batches you could even use a stick.|
|Airid||Optional, but recommended. This will help eliminate air bubbles in your castings.|
|Plaster or Hydrocal||We sell these to accommodate our customers, but you may wish to source it locally since the shipping cost is high due to the weight. Plaster may be found in most home centers and hardware stores. Hydrocal is much harder to find locally.|
|Scale||Optional. Plaster can be mixed by guesswork, but a scale will allow good repeatable results every time. Any moderately accurate scale that can weigh in 1 ounce increments should suffice.|
|Hangers||If making wall hangings you will need some hangers.|
|Pigment||Optional. Useful if you desire to color the plaster without using paint.|
|Rice Bag or Sand Box||Most plaster molds do not sit flat and it will be necessary to support them. More details on this will be explained below|
|Knife||I use a standard utility knife but any semi sharp knife will suffice.|
|Toothpicks||To support hangers if necessary.|
Cover your work surface with newspaper or a piece of vinyl. As you can see from this photo plaster casting is a bit messy so it is important to protect your countertops.
Inspect the mold to make sure it is clean and dry - any dirt will show up on the finished casting.
Most molds will not sit flat on the counter so it is necessary to support them for use. I find the simplest way is to use a zip lock bag with a few pounds of rice in it. Rice bags are easy to pack and store when not in use, and made with common materials most folks have handy. A box of sand works well also, but is more difficult to make and store when not in use.
The surface tension of the water in plaster tends to trap air causing pinholes in the finished casting. Airid is a product designed to break that surface tension, minimizing the chance of trapped air. Spray or wipe a thin coat of Airid into the mold (optional).
Wiggle the mold down onto your rice bag or sandbox until it appears level. You are now ready to begin mixing your plaster.
Click Here To Continue To Part 2 - Mixing Plaster
Disclaimer: The information presented here is accurate to the best of my knowledge and common plaster crafting practices as of the time of this writing. Originally published in September 2006 and modified in April 2012. The author and the publisher accept no liability for the use or misuse of any of the information presented in this article. This article is presented for informational purposes and is used at your own risk.
Author: Bob Sherman
Publisher: Bobby's Craft Boutique Inc.
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