To make Carnival Glass, you begin the process as with regular soda-lime glass. You mix sodium carbonate (soda) with lime, silicon dioxide (silica) and other chemicals in a furnace and melt it to a molten state. If the color of the glass is to be anything but clear, coloring is added to the mix. Then you pour your mixture into a mould and let it take on the distinct shape and design. Once it is cooled and molded the glass is removed from the mould and then reheated to add any additional features, such as a ruffled edge.
Finally, the trick for the iridescent finish is sprayed over the glass while it is still hot. The spray is composed of liquid metallic salts and the process of applying it is termed “doping.” Metallic salt is the technical term for a compound composed of a metal and nonmetal. They are ionic compounds, meaning they are held together by ionic interactions (positive and negative charges). Regular table salt is an example of a metallic salt – composed of Sodium (Na+) and Chloride (Cl–). A few examples of the metallic salts used to provide iridescence were potassium phosphate, calcium fluoride, and sodium fluoraluminate.
You may hear rumors regarding the toxicity of carnival glass due to the metallic sprays. Though some of the chemicals used can be dangerous, they are not so once on the glass as a finished product. However, in the article “A Lesson in Toxicity” by Dave Shetlar he states “Doping…is a very different issue. Iron chloride and tin chloride when put into “mild” acids can produce some pretty toxic fumes when they hit hot glass. In early factories, if the dope was sprayed without the use of a fume hood (as is done today), the doper and handlers may have been exposed to some pretty dangerous fumes. However, I suspect that it was not really much worse than what was happening in the iron smelting, and steel manufacturing factories of the same period!”
Once the glass has been sprayed it is placed on a lehr. This is a special machine composed of a covered conveyor belt. The glass piece is placed on the belt and it passes very slowly through the enclosed area, allowing it to cool at a slow rate. This prevents any temperature related stressors which could cause the glass to crack or lower its integrity. For a whole 24 hour period the glass moves along the lehr until it is finally cooled to room temperature.
Each of the carnival glass producing companies had distinct characteristics for their works. It is therefore possible to discover where and what time frame a particular piece came from. As discussed in our article on the History of Carnival Glass, the two main factors distinguishing one work of carnival glass from another are the pattern and the color of the glass. When describing the color of carnival glass it is done in reference to the color of the actual glass upon which the metallic spray was added – not the color of the iridescent metallic spray. For many pieces this can be determined by looking at the bottom, where often no spray was applied. So when a piece is referred to as blue, it may not even look blue; but it is called so because the original glass without the metallic spray was blue. The only exceptions to this are marigold, clambroth, white, and some smoke which are clear glass.
The main manufacturers of carnival glass were: Fenton, Dugan, Northwood, Millersburg and Imperial. To identify a particular piece it is necessary to match the color and pattern to a known pattern of one of the manufacturers. There are extensive libraries listing the 2,000+ patterns that were used. It can be a cumbersome task, but based on the growing number of collectors – a rewarding one as well! Please visit carnivalglass101.carnivalheaven.com and ddoty.com for more information and resources on the spectacular world of Carnival Glass.