Based on the information you provided, the Duke 40gm soldering wire sounds like a possible lead-based solder. Here's why:
* **Brand information:**  While there isn't a lot of widespread information available online about "Duke" soldering wire specifically, the lack of mention of lead-free properties suggests it might be a traditional lead-based solder.
* **Weight:**  40 grams is a common weight for lead-based solder spools.
However, it's important to consider lead safety and regulations:
* **Lead dangers:** Lead exposure can be harmful to health, especially for children and pregnant women.  
* **Lead-free alternatives:**  Lead-free solders are widely available and offer safer alternatives for electronic soldering. 
Here are some recommendations:
* **Check the packaging:** If you still have the packaging for the Duke soldering wire, look for any mention of lead content or if it's lead-free.
* **Consider lead-free options:** If safety is a concern,  especially if you'll be soldering around children or in poorly ventilated areas,  it's best to use lead-free solder. Lead-free solders typically have a higher melting point than lead-based solders, so you might need to adjust your soldering iron temperature accordingly.
* **Alternatives:** If you can't find any information on the lead content of your Duke wire, it might be best to err on the side of caution and use a lead-free solder.  Popular lead-free solder alloys include SAC (Tin-Silver-Copper) and SnCu (Tin-Copper).
Here are some additional points to consider:
* **Application:**  If you're soldering for hobbyist electronics or personal projects, lead-free solder is generally a good choice.
* **Regulations:**  Some countries or industries may have regulations restricting the use of lead-based solders.
If you decide to use your Duke soldering wire, be sure to work in a well-ventilated area and wash your hands thoroughly after soldering.  However, for most applications, lead-free solder is the safer and more widely recommended option.